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Growth Hormone (GH; Somafotropin) Deficiency

GH is a peptide that is secreted by the acidophil cells of the pituitary. GH release depends on the interplay between two opposing hormones secreted by the hypothalamus; growth hormone-releasing hormone (GHRH), which stimulates GH release, and somatostatin, which inhibits GH release. GH exerts its effect through stimulation of various tissues to produce a group of peptide growth factors called “somatomedins.” The most important somatomedin is somatomedin-C, which is produced by the liver and which acts on cartilage. High levels of somatomedin participate in a GH feedback mechanism by inhibiting pituitary release of GH and also by stimulating hypothalamic secretion of somatostatin. In addition, there apparently are numerous other factors that influence GH production to varying degree, some of which act through the hypothalamus and others that apparently do not. Normal GH secretion takes place predominantly (but not entirely) during sleep in the form of short irregularly distributed secretory pulses or bursts. GH deficiency most often is suspected or questioned as part of the differential diagnosis of retarded growth or short stature. A considerable number of hormones and factors are necessary for adequate growth. Some of the most important are listed in the box on this page. Etiologies of short stature (whose extreme result is dwarfism) are listed in the box. Diagnosis of many of these conditions is discussed elsewhere. Deficiency of GH will be discussed in the context of pediatric or childhood growth retardation. As noted previously, GH deficiency may be part of multihormone pituitary dysfunction but may exist as an isolated defect in an otherwise apparently normal pituitary.

GH deficiency tests

Growth hormone assay. The most widely used screening test for GH deficiency is serum GH assay. Values are elevated by sleep, exercise of any type, and various foods. Therefore, if a single basal level is obtained, the specimen should be secured in the morning after an overnight fast, and the patient should be awake but not yet out of bed. Because basal normal range for single specimens includes some normal persons with values low enough to overlap the range of values found in patients with GH deficiency, only a result high enough to rule out GH deficiency is significant. Low results either may be due to GH deficiency or may simply be low-normal. Other conditions that increase GH levels besides those previously mentioned include severe malnutrition and chronic liver or renal disease. Patients with psychiatric depression or poorly controlled diabetes mellitus secrete more GH than normal persons during waking hours but not during sleep.

Body Factor Snecessary for Normal Growth
GH
Somatomedins
Insulin
Thyroxine
Sex hormones
Adequate nutritional factors
Partial List of Short Stature Etiologies
Familial short stature
Constitutional growth delay
Turner’s syndrome
Trisomy syndromes (e.g., Down’s syndrome)
GH deficiency (isolated or hypopituitarism)
Hypothyroidism
Severe malnutrition
Malabsorption
Uncontrolled type I diabetes mellitus
Severe chronic illness with organ failure (chronic renal disease, congenital heart disease, severe chronic hemolytic anemia)
Excess androgens or estrogens
Excess cortisol (Cushing’s syndrome or iatrogenic)
Psychosocial retardation (abused or severely emotionally deprived children)

Two relatively simple screening tests for GH deficiency have been advocated. It has been found that a major part of daily GH secretion takes place in a short period beginning approximately 1 hour after onset of deep sleep (electro-encephalogram [EEG] stage 3 or 4). Therefore, one method is to observe the patient until deep sleep occurs, wake the patient 60-90 minutes later, and quickly draw a blood sample for GH assay. About 70% of normal children or adults have GH values sufficiently elevated (і7 ng/ml; 7 µg/L) to exclude GH deficiency. Strenuous exercise is a strong stimulus to GH secretion. An exercise test in which the patient exercises vigorously for 20-30 minutes and a blood sample is drawn for GH assay 20-30 minutes after the end of the exercise period has been used. About 75%-92% of normal children have sufficiently elevated levels of GH after exercise to exclude GH deficiency.

A third method involves integrated multihour time period blood specimens; this method is being used in research and in some medical centers. Because of the irregular pulsatile nature of GH secretion, it was found that a more reliable estimate of GH secretion levels could be obtained by drawing blood specimens for GH assay every 20-30 minutes from an indwelling catheter over a 24-hour period. Some investigators report that the same information can be obtained using an 8 P.M. -8 A.M. 12-hour period while the patient is sleeping. The integration (averaging) of the GH specimen values correlate much better with clinical findings, especially in patients with partial GH deficiency (also known as “neurosecretory GH deficiency”), than does a single-specimen value. This test is being used when standard tests are equivocal or normal in spite of clinical suspicion of GH deficiency.

Somatomedin-C assay. Somatomedin-C assay has been proposed as a screening test for GH deficiency, pituitary insufficiency, and acromegaly. As noted previously, somatomedin serum levels depend on serum GH levels, and it has been postulated that the growth-promoting actions of GH are actually carried out by the somatomedins. Somatomedin-C is produced in the liver and circulates in serum predominantly bound to certain long-lived high molecular weight serum proteins. Normal values for females are about 10%-20% higher than those for males in both children and adults. Normal values also depend on age and developmental stage. There is a considerable increase during the pubertal adolescent growth spurt. Radioreceptor and immunoassay measurement techniques are available in large reference laboratories for somatomedin-C. Serum levels are low in most patients with isolated GH deficiency or pituitary insufficiency. The assay has some advantages in that the result does not depend on relation to food intake or time of day. Disadvantages are that only large medical centers or reference laboratories offer the assay and that various conditions besides pituitary dysfunction may produce a low result. These nonpituitary conditions include decreased caloric intake and malnutrition, malabsorption, severe chronic illness of various types, severe liver disease, hypothyroidism, and Laron dwarfism. Therefore, a clearly normal result is more diagnostic than a low result, since the low result must be confirmed by other tests. Somatomedin-C levels are elevated in acromegaly (discussed later).

Growth hormone stimulation tests. Since only a clearly normal single-specimen GH result excludes pituitary GH secretion deficiency, in patients with lower single-specimen GH values a stimulation test is needed for detection of deficiency or confirmation of normality. The classic procedures are insulin-induced hypoglycemia and arginine infusion. Pituitary insufficiency cannot be documented on the basis of pituitary nonresponse to either test agent alone, since about 20% of normal persons may fail to respond satisfactorily to insulin alone or to arginine alone. Estrogen administration tends to increase GH secretion, and some investigators have obtained better results from arginine infusion after administration of estrogens.

Stimulation tests with other substances, including glucagon, tolbutamide, levodopa, and clonidine, have been proposed. Of these, levodopa stimulation seems to give best results; some believe that it has an accuracy equal to or better than that achieved with insulin or arginine stimulation and, in addition, is easier and safer to perform. One investigator found a greater number of unreliable results in older persons and depressed patients. Blood samples are drawn 60 and 90 minutes after oral levodopa administration. Investigators have explored various means to improve the accuracy of these tests. Sympathetic nervous system alpha-adrenergic stimulants enhance GH release while beta stimulation is inhibitory. The beta blocker propranolol, given with levodopa, enhances levodopa stimulation of GH secretion and improves identification of normal persons. For example, one investigator reported only about 5% false low GH results with the combination of levodopa and propranolol but 20% false low results with levodopa alone. Clonidine stimulation has attracted interest because it has fewer side effects and is more convenient to perform than the other tests. However, there is disagreement regarding test accuracy, partially because some investigators used lower doses that did not provide maximal effect. Overall effectiveness with higher doses is said to be similar to that of the other stimulation tests. The major problems with the stimulation tests are the need for more than one test to confirm a failure to stimulate GH levels and the existence of partial (neurosecretory) GH secretion deficiency, in which GH secretion is not sufficient for normal bone growth but stimulation test results are normal. At present, the diagnosis of partial GH deficiency is made through 12-hour or 24-hour integrated GH measurements or by therapeutic trial with GH. GHRH is now available for investigational use. Preliminary reports using GHRH as a stimulation test in GH deficiency have yielded results rather similar to those of standard stimulatory tests. GHRH testing may be useful in differentiating hypothalamic dysfunction from pituitary GH dysfunction. However, some patients with hypothalamic dysfunction may show apparent failure to respond to initial stimulation of the pituitary by GHRH, thus simulating pituitary GH secretory dysfunction, although they respond normally after repeated stimulation.

Bone X-ray studies. Hand-wrist x-ray studies for bone age (bone maturation) are strongly recommended in patients with suspected growth disorders. The actual genetic sex (which in some instances may be different from the apparent sex) must be furnished to the radiologist so that the correct reference range may be used. Bone maturation should be compared with linear growth (height) and chronologic age. The box on this page lists some conditions associated with retarded or accelerated bone age.

December 24, 2009
Pituitary Insufficiency

Body organs affected by stimulatory hormones from the pituitary include the thyroid, adrenals, and gonads. Bone growth in childhood is dependent on pituitary GH. Pituitary failure does not produce a clear-cut syndrome analogous to syndromes produced by failure of pituitary-controlled organs (“end organs”) such as the thyroid. Therefore, pituitary hormone deficiency is considered only when there is deficiency-type malfunction in one or more end organs or metabolic processes such as growth that are dependent on pituitary hormones. Diagnosis is complicated by the fact that primary end organ failure is much more common than pituitary hormone deficiency. Another source of confusion is that most abnormal effects that can be produced by pituitary dysfunction can also be produced or simulated by nonpituitary etiologies. In addition, the hypothalamus controls pituitary secretion of several hormones, so that hypothalamic abnormality (e.g., defects produced by a craniopharyngioma or other hypothalamic tumor) can result in pituitary dysfunction.

Pituitary insufficiency in adults most commonly results in deficiency of more than one pituitary hormone and is most frequently caused by postpartum hemorrhage (Sheehan’s syndrome). Pituitary tumor is also an important cause. Gonadal failure is ordinarily the first clinical deficiency to appear. It is followed some time later by hypothyroidism. The first laboratory abnormality is usually failure of the GH level to rise normally in response to stimulation.

Diagnosis

Diagnosis of pituitary insufficiency can be made by direct or indirect testing methods. Indirect methods demonstrate that a hypofunctioning organ that is pituitary dependent shows normal function after stimulation by injection of the appropriate pituitary hormone. Direct methods consist of blood level assays of pituitary hormones. Another direct method is injection of a substance that directly stimulates the pituitary. Now that pituitary hormone assays are available in most medical centers and sizable reference laboratories, indirect tests are much less frequently needed.

Assay of pituitary hormones. Pituitary hormones are peptide hormones rather than steroid hormones. Currently available immunoassays are a great improvement over original bioassays and even the first-generation radioimmunoassays (RIAs). However, with the exception of thyroid-stimulating hormone (TSH), pituitary hormone assays are still ordered infrequently, so that most physician’s offices and ordinary hospital laboratories do not find it economically worthwhile to perform the tests. The assays for TSH and adrenal cortex stimulating hormone (adrenocorticotropin; ACTH) are discussed in the chapters on thyroid and adrenal function. Pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) assay are discussed later. GH deficiency is probably the most frequent pituitary hormone deficiency, either in overall pituitary failure or as an isolated defect leading to growth retardation in childhood. GH assay will be discussed in detail in relation to childhood growth disorders. GH assay has been used as an overall screen for pituitary insufficiency, but not all cases of pituitary hormone secretion deficiency have associated GH secretion deficiency. Prolactin is another pituitary hormone, but prolactin secretion is one of the last pituitary functions to disappear when the pituitary is injured.

In primary end organ failure the blood level of stimulating hormone from the pituitary is usually elevated, as the pituitary attempts to get the last possible activity from the damaged organ. Therefore, in the presence of end organ failure, if values for the pituitary hormone that stimulates the organ are in the upper half of the reference range or are elevated, this is strong evidence against deficiency of that pituitary hormone. Theoretically, inadequate pituitary hormone secretion should result in serum assay values less than the reference range for that pituitary hormone. Unfortunately, low-normal or decreased values of some pituitary hormones may overlap with values found in normal persons, at least using many of the present-day commercial assay kits. In that situation, stimulation or suppression testing may be necessary for more definitive assessment of pituitary function.

Stimulation and suppression tests. Pituitary suppression and stimulation tests available for diagnosis of pituitary hormone secretion deficiency include the following:

1. Metyrapone (Metopirone) test based on the adrenal cortex-pituitary feedback mechanism involving ACTH and cortisol but dependent also on hypothalamic function.
2. Thyrotropin-releasing hormone (TRH) test involving the ability of synthetic TRH to stimulate the pituitary by direct action to release TSH and prolactin.
3. Tests involving pituitary gonadotropins (LH and FSH) such as clomiphene stimulation (involving the hypothalamic-pituitary axis) or LH-releasing hormone (LHRH) administration (direct pituitary stimulation).
4. Tests of hypothalamic function, usually based on establishing pituitary normality by direct stimulation of the pituitary (TRH stimulation of TSH and prolactin, LHRH stimulation of LH secretion, etc.) followed by use of a test that depends on intact hypothalamic function to stimulate pituitary secretion of the same hormone. An example is the use of TRH to stimulate prolactin secretion by the pituitary, followed by chlorpromazine administration, which blocks normal hypothalamic mechanisms that inhibit hypothalamic stimulation of pituitary prolactin secretion and which leads to increased prolactin secretion if the hypothalamus is still capable of stimulating prolactin release.

December 24, 2009
Adrenal and Nonadrenal Causes of Hypertension

Cushing’s syndrome, primary aldosteronism, unilateral renal disease (rarely, bilateral renal artery stenosis), and pheochromocytoma often produce hypertension. Hypertension due to these diseases is classified as secondary hypertension, in contrast to primary idiopathic (essential) hypertension. Although patients with these particular diseases that cause secondary hypertension are a relatively small minority of hypertension patients, the diseases are important because they are surgically curable. The patient usually is protected against the bad effects of hypertension by early diagnosis and cure. Patients who must be especially investigated are those who are young (less than age 50 years), those whose symptoms develop over a short time, or those who have a sudden worsening of their hypertension after a previous mild stable blood pressure elevation.

December 22, 2009
Adrenal Medulla Dysfunction

The only syndrome in this category is produced by pheochromocytomas. Pheochromocytoma is a tumor of the adrenal medulla that frequently secretes epinephrine or norepinephrine. This causes hypertension, which may be continuous (about 30% of patients) or in short episodes (paroxysmal). Although rare, pheochromocytoma is one of the few curable causes of hypertension and so should be considered as a possible etiology in any patient with hypertension of either sudden or recent onset. This is especially true for young or middle-aged persons.

Approximately 90% of pheochromocytomas in adults arise in the adrenal (more often in the right adrenal). Of the 10% that are extraadrenal, the great majority are located in the abdomen, with 1%-2% in the thorax and neck. Extraadrenal abdominal tumors are usually found in the paraaortic sympathetic nerve chain (below the level of the renal artery), but perhaps one third are located in the remnants of the organ of Zuckerkandl (which is situated in the paraaortic region between the origin of the inferior mesenteric artery and the bifurcation of the aorta). About 20% of pheochromocytomas are multiple and about 10% are bilateral. Approximately 5%-10% (range, 3%-14%) of all pheochromocytomas are malignant, but the malignancy rate for extraadrenal abdominal tumors is reported to be about 25%-30%. Although hypertension is the most common significant clinical finding, 10%-20% of pheochromocytomas do not produce hypertension (Table 30-3). Hyperglycemia is reported in about 50% of patients. In one autopsy series of 54 cases, only 25% were diagnosed during life.

Table 30-3 Common signs and symptoms of pheochromocytoma

About 5% of pheochromocytoma patients have neurofibromatosis, and in 5%-10% the pheochromocytoma is associated with the multiple endocrine neoplasia (MEN) syndrome type II (also known as “IIA,” with medullary carcinoma of the thyroid) or III (also known as “IIB,” with mucosal neuromas). The MEN syndrome pheochromocytomas are bilateral in 50%-95% of cases and multiple in about 70% of cases, whereas nonfamilial (sporadic) pheochromocytomas are usually unilateral and are multiple in about 20% of cases. About 5% of the familial cases are said to be malignant.

In children, extraadrenal pheochromocytomas are more common (30% of cases), more often bilateral (25%-70% of cases), and more often multiple (about 30% of cases) than in adults. About 10%-20% of adrenal or extraadrenal childhood pheochromocytomas are reported to be malignant.

Tests for pheochromocytoma

Regitine test. The first tests for pheochromocytomas were pharmacologic, based on neutralization of epinephrine effects by adrenergic-blocking drugs such as Regitine. After basal blood pressure has been established, 5 mg of Regitine is given intravenously, and the blood pressure is checked every 30 seconds. The result is positive if systolic blood pressure decreases more than 35 mm Hg or diastolic pressure decreases 25 mm Hg or more and remains decreased 3-4 minutes. Laboratory tests have proved much more reliable than the pharmacologic procedures, which yield an appreciable percentage of false positives and negatives.

Clonidine suppression test. Clonidine is a centrally acting alpha-adrenergic agonist that inhibits sympathetic nervous system catecholamine release from postganglionic neurons. The patients should not be on hypertensive medication (if possible) for at least 12 hours before the test. After a baseline blood specimen is obtained, a single 0.3-mg oral dose of clonidine is administered and a second blood specimen is drawn 3 hours later. Most patients without pheochromocytoma had postdose plasma norepinephrine values more than 50% below baseline and plasma catacholamine values less than 500 pg/ml. Most patients with pheochromocytoma showed little change in plasma norepinephrine values between the predose and postdose specimens and had a plasma catacholamine postdose level greater than 500 pg/ml. Apparently, better results are obtained when baseline urine norepinephrine values are greater than 2000 pg/ml (86%-99% sensitivity) than when the baseline value is less than 2000 pg/ml (73%-97% sensitivity). Medication such as tricyclic antidepressants, thiazide diuretics, and beta blockers may interfere with the test.

Catecholamine/vanillylmandelic acid/metanephrine assay. The catecholamines epinephrine and norepinephrine are excreted by the kidney, about 3%-6% free (unchanged), and the remainder as various metabolites. Of these metabolic products, the major portion is vanillylmandelic (vanilmandelic) acid (VMA), and the remainder (about 20%-40%) is compounds known as metanephrines. Therefore, one can measure urinary catecholamines, metanephrines, or VMA. Of these, urine metanephrine assay is considered by many to be the most sensitive and reliable single test. There are also fewer drugs that interfere.

Most investigators report that urine metanephrine assay detects about 95% (range, 77%-100%) of patients with pheochromocytoma. Sensitivity of urine catecholamines is approximately 90%-95% (range, 67%-100%) and that of urine VMA assay is also about 90%-95% (range, 50%-100%). One report indicates that metanephrine excretion is relatively uniform and that a random specimen reported in terms of creatinine excretion can be substituted for the usual 24-hour collection in screening for pheochromocytoma. Methylglucamine x-ray contrast medium is said to produce falsely normal urine metanephrine values for up to 72 hours. One report found that 10%-15% of mildly elevated urine metanephrine values were falsely positive due to medications (such as methyldopa) or other reasons.

Although the metanephrine excretion test is slowly gaining preference, VMA and catecholamine assay are still widely used. All three methods have detection rates within 5%-10% of one another. A small but significant percentage of pheochromocytomas are missed by any of the three tests (fewer by metanephrine excretion), especially if the tumor secretes intermittently. The VMA test has one definite advantage in that certain screening methods are technically more simple than catecholamine or metanephrine assay and therefore are more readily available in smaller laboratories. The VMA screening methods are apt to produce more false positive results, however, so that abnormal values (or normal results in patients with strong suspicion for pheochromocytoma) should be confirmed by some other procedure.

Catecholamine production and plasma catecholamine levels may be increased after severe exercise (although mild exercise has no appreciable effect), by emotional stress, and by smoking. Uremia interferes with assay methods based on fluorescence. Other diseases or conditions that may increase plasma or urine catecholamines or urine catecholamine metabolites are hypothyroidism, diuretic therapy, heavy alcohol intake, hypoglycemia, hypoxia, severe acidosis, Cushing’s syndrome, myocardial infarction, hemolytic anemia, and occasionally lymphoma or severe renal disease. In addition, bananas, coffee, and various other foods as well as certain medications may produce falsely elevated results. These foods or medications produce appreciable numbers of false-positive results in some of the standard screening techniques for VMA. An abnormal result with the “screening” VMA techniques should be confirmed by some other VMA method. Although other VMA methods or methods for metanephrine and catecholamine assay are more reliable, they too may be affected by certain foods or medications, so it is best to check with individual laboratories for details on substances that affect their particular test method. Reports differ on whether some patients with essential hypertension may have elevated serum or urine catecholamine or catecholamine metabolite results; and if so, how often it occurs and what percentage are due to conditions known to elevate catecholamines, or to medications, or to unknown causes.

Some investigators use urine fractionated catecholamines (epinephrine and norepinephrine, sometimes also dopamine) by high-pressure liquid chromatography as a confirmatory test for pheochromocytoma. It is said that about 50%-70% of pheochromocytomas produce epinephrine, about 75%-85% produce norepinephrine, and about 95% produce one or the other.

Plasma catecholamine assay. Several investigators report better sensitivity in pheochromocytoma with plasma catecholamine assay than with the particular urine metabolite assays they were using. Another advantage is the simplicity of a single blood specimen versus 24-hour collection. However, upright posture and stress can greatly affect plasma catecholamine levels, even the stress of venipuncture, so it is best to draw the specimen in the early morning before the patient rises. Even then, some advocate placement of an indwelling venous catheter or heparinized scalp vein apparatus with an interval of 30 minutes between insertion of the catheter and withdrawal of the blood specimen. One investigator reported that plasma catecholamine values decreased rapidly if the red blood cells (RBCs) were not removed within 5 minutes after obtaining the specimen. Placing the specimen on ice was helpful but only partially retarded RBC catecholamine uptake. Plasma catecholamine assay detection rate in pheochromocytomas is about 90% (literature range, 53%-100%). Failure to detect the tumor in some instances is due to intermittent tumor secretion. Urine collection has the advantage of averaging the 24-hour excretion.

Tumor localization methods. Once pheochromocytoma is strongly suspected by results of biochemical testing, certain procedures have been useful in localizing the tumor. Intravenous pyelography with nephrotomography is reported to have an accuracy of about 70% in detecting adrenal pheochromocytoma. CT has been reported to detect 85%-95% (range, 84%-over 95%) of pheochromocytomas, including some in extraadrenal locations. Consensus now is that CT (or magnetic resonance imaging [MRI]) is the best single localization test (better than ultrasound). Radioactive mIBG has been used to locate pheochromocytomas and other neural tumors with sensitivity of about 85%. However, this procedure is only available in a very few nuclear medicine centers. Angiographic procedures are reported to detect 60%-84% of pheochromocytomas; but angiography is somewhat controversial because it is invasive, because it yields about 10% false negative results in adrenal tumors, and because some investigators believe that the incidence of tumor bilaterality warrants exploration of both sides of the abdomen regardless of angiographic findings.

Miscellaneous procedures. Serum chromogranin A (CgA), a substance produced in the adrenal medulla and some other endocrine and neuroendocrine tissues and tumors, has been used in diagnosis of pheochromocytoma. CgA is not affected by posture, venipuncture, and many medications that interfere with catecholamine assay; in one series CGA detected 86% of pheochromocytomas.

Deoxyribonucleic acid (DNA) analysis by flow cytometry in small numbers of patients suggest that pheochromocytomas with diploid ploidy are most often (but not always) benign, whereas those that are aneuploid are most often (but not always) malignant.

Serum neuron-specific enolase (NSE) in small numbers of patients suggest that pheochromocytomas with normal NSE levels are usually benign, but those with elevated values are often malignant.

December 22, 2009
Hyporeninemic Hypoaldosteronism

Another adrenal cortex deficiency disease is hyporeninemic hypoaldosteronism. Since this condition is usually due to a deficiency of renin, the disorder is a secondary rather than a primary defect in aldosterone secretion. The characteristic laboratory feature is hyperkalemia, and the condition is usually discovered when a patient is found to have hyperkalemia not due to any other cause. The serum sodium level is normal or mildly decreased. There may be a mild metabolic acidosis. If dietary sodium is restricted, renal salt wasting develops, since deficiency of aldosterone makes it difficult to conserve sodium. Hyperkalemia is not diagnostic of hyporeninemic hypoaldosteronism since it may be found in Addison’s disease, salt-wasting congenital adrenal hyperplasia, renal failure, and other conditions listed in the chapter on serum electrolytes.

December 22, 2009
Secondary Adrenal Insufficiency

Secondary adrenal insufficiency is due to deficient production of ACTH by the pituitary. This usually results from pituitary disease but is occasionally due to hypothalamic disorders. In general, primary adrenocortical insufficiency (Addison’s disease) is associated with inability of the adrenal to produce either cortisol or aldosterone. In addition, there may be cutaneous hyperpigmentation due to pituitary hypersecretion of melanocytic-stimulating hormone (MSH). In secondary adrenal insufficiency, aldosterone secretion is usually sufficient to prevent hyperkalemia, and hyponatremia tends to be less severe. Cutaneous pigmentation does not occur because the pituitary does not produce excess MSH. The major abnormality is lack of cortisol due to deficiency of ACTH. The laboratory picture may simulate inappropriate ADH syndrome.

Differentiation of primary from secondary adrenal insufficiency includes plasma ACTH assay (typically increased levels in primary Addison’s disease and normal or low in pituitary or hypothalamic etiology disease) and in some cases, the prolonged ACTH test. Other tests for pituitary function (metapirone test or CRH test) may be useful in some patients.

December 22, 2009
Addison’s Disease

Addison’s disease is primary adrenocortical insufficiency from bilateral adrenal cortex destruction. Tuberculosis used to be the most frequent etiology but now is second to autoimmune disease atrophy. Long-term steroid therapy causes adrenal cortex atrophy from disuse, and if steroids are abruptly withdrawn, symptoms of adrenal failure may develop rapidly. This is now the most common cause of addisonian-type crisis. Less common etiologies of Addison’s disease are infection, idiopathic hemorrhage, and replacement by metastatic carcinoma. The most frequent metastatic tumor is from the lung, and it is interesting that there often can be nearly complete replacement without any symptoms.

The salt-wasting forms of congenital adrenal hyperplasia—due to congenital deficiency of certain enzymes necessary for adrenal cortex hormone synthesis—might also be included as a variant of Addison’s disease.

Weakness and fatigability are early manifestations of Addison’s disease, often preceded by infection or stress. Other signs and symptoms of the classic syndrome are hypotension of varying degree, weight loss, a small heart, and sometimes skin pigmentation. Anorexia, nausea, and vomiting occur frequently in adrenal crisis. The most common symptoms of adrenal crisis are hypotension and nausea.

General laboratory tests

Serum sodium is decreased in 50%-88% of patients with primary Addison’s disease, and serum potassium is mildly elevated in 50%-64% of cases (due to concurrent aldosterone deficiency). One investigator reported hypercalcemia in 6% of patients. There occasionally may be a mild hypoglycemia, although hypoglycemia is more common in secondary adrenal insufficiency. Serum thyroxine is low normal or mildly decreased and TSH is upper normal or mildly increased. Plasma aldosterone is usually decreased and plasma renin is elevated. There often is a normochromic-normocytic mild anemia and relative lymphocytosis with a decreased neutrophil count. Total eosinophil count is usually (although not always) close to normal.

In primary adrenal insufficiency, a morning serum cortisol value is typically decreased and the plasma ACTH value is increased. Arginine vasopressin (AVP or ADH) is usually elevated.

Diagnostic tests in Addison’s disease

Screening tests. A single random serum cortisol specimen has been used as a screening procedure, since theoretically the value should be very low in Addison’s disease and normal in other conditions. Unfortunately, there are sufficient interfering factors so that its usefulness is very limited. Because serum cortisol normally has a circadian rhythm with its peak about 6-8 A.M., the specimen must be drawn about 6-8 A.M. in order not to misinterpret a lower value drawn later in the day. Stress increases plasma cortisol levels, although the increase is proportionately much less in Addison’s disease than in normal persons. The classic patient with Addison’s disease in crisis has an early morning cortisol level of less than 5 µg 100 ml (138 nmol/L), and a level of 5-10 µg/100 ml (138-276 nmol/L) is suspicious for Addison’s disease, especially if the patient is under stress. Patients with milder degrees of illness or borderline deficiency of cortisol may have a morning cortisol value of more than 10 µg/100 ml. It is often difficult to determine whether mild elevation of more than 10 µg/100 ml is due to stress or is a normal level. An early morning cortisol level of more than 20 µg/100 ml (550 mmol/L) is substantial evidence against Addison’s disease. Many endocrinologists do not consider a single random cortisol level to be reliable in screening for Addison’s disease. In spite of this it is usually worthwhile to obtain a serum specimen early for cortisol assay for diagnostic purposes (if it excludes Addison’s disease) or as a baseline (if it does not). A plasma sample should be obtained at the same time (EDTH anticoagulant) and frozen in case ACTH assay is needed later. As noted previously, serum sodium (and also chloride) is often low in classic cases, and if so would be suggestive of Addison’s disease if it were associated with a normal or elevated urine sodium and chloride level. However, as noted previously, serum sodium can be within population reference range in 12%-50% of patients.

Rapid ACTH screening (“Cortrosyn”). Most investigators now prefer a rapid ACTH stimulation test rather than the single cortisol assay, since the rapid test can serve as a screening test unless the patient is extremely ill and in some patients may provide the same information as a confirmatory test. After a baseline serum cortisol specimen is obtained, 25 units of ACTH or 0.25 mg of corsyntropin (Cortrosyn or Synacthen, a synthetic ACTH preparation) is administered. There is variation in technique among the descriptions in the literature. Most measure plasma cortisol response after administration of corsyntropin but a few assay urinary 17-OHCS. Some inject corsyntropin intramuscularly and others intravenously. Intravenous (IV) administration is preferred but not required under ordinary circumstances. If the patient is severely ill or is hypotensive, IV is recommended to avoid problems in corsyntropin absorption. Some obtain a serum cortisol specimen 30 minutes after giving corsyntropin, whereas others do so at 60 or 120 minutes. Some obtain samples at two intervals instead of one. The majority appear to obtain one sample at 60 minutes. Some also obtain a sample at 30 minutes; this helps confirm the 60-minute value and avoids technical problems. However, the 30-minute specimen is not considered to be as reliable as the 60-minute specimen, especially if intramuscular (IM) injection was used. Theoretically, patients with primary adrenal insufficiency should demonstrate little response, whereas patients with pituitary insufficiency or normal persons should have stimulated cortisol levels that exceed 20 µg/100 ml (550 mmol/L). Some endocrinologists require an increment of at least 7 µg above baseline in addition to a peak value of 20 µg or more, especially when baseline cortisol is over 10µg/100 ml (225 mmol/L). However, increments less than or greater than 7µg are not as reproducible (on repeat corsyntropin tests) as the 20-µg peak cutoff value. Some patients with pituitary insufficiency demonstrate normal response to corsyntropin and some have a subnormal response. Because corsyntropin test results are not uniform in patients with pituitary insufficiency, it has been suggested that aldosterone should be measured as well as cortisol. Aldosterone levels should increase in pituitary hypofunction but should not rise significantly in primary adrenal failure. The metyrapone test is also useful to diagnose pituitary insufficiency.

Some patients may have equivocal rapid test results, and others may have been treated with substantial doses of steroids for considerable periods of time before definitive tests for etiology of Addison’s disease are attempted. Under long-term steroid suppression, a normal adrenal cortex may be unable to respond immediately to stimulation. A definitive diagnosis of Addison’s disease is possible using prolonged ACTH stimulation. The classic procedure is the 8-hour infusion test. If biologic rather than synthetic ACTH is used, many recommend giving 0.5 mg of dexamethasone orally before starting the test to prevent allergic reactions. A 24-hour urine specimen is taken the day before the test. Twenty-five units of ACTH in 500 ml of saline is given intravenously during an 8-hour period while another 24-hour urine specimen is obtained. In normal persons, there will be at least a twofold to fourfold increase in cortisol or 17-OHCS levels. In Addison’s disease, there is practically no response. If pituitary deficiency is suspected, the test should be repeated the next day, in which case there will be a gradual, although relatively small, response. If exogenous steroids have been given over long periods, especially in large doses, the test period the classic approach is to repeat the 8-hour ACTH infusion procedure daily for 5-7 days. Patients with primary Addison’s disease should display little daily increment in cortisol values; those with secondary Addison’s disease should eventually produce a stepwise increase in cortisol values. Some have used a continuous ACTH infusion for 48 hours or depot IM synthetic ACTH preparations once daily instead of IV infusions or standard IM injections twice daily. If the patient has symptoms of adrenal insufficiency, both the rapid ACTH test and the prolonged ACTH test can be performed while the patient is taking 0.5-1.0 mg of dexamethasone per day, as long as therapy has not extended more than 5-6 days before starting the tests. Dexamethasone can be used because at these low doses it will not be a significant part of either serum or urine cortisol assays. Long periods of glucocorticoid therapy will interfere with the pituitary-adrenal feedback response necessary for rapid cortisol response to ACTH and will require longer periods of ACTH stimulation in the prolonged ACTH stimulation test.

Thorn Test. If steroid measurements are not available, the Thorn test could be substituted, although it is not as accurate. First, a count of total circulating eosinophils is done. Then the patient is given 25 units of ACTH, either intravenously in the same way as in the ACTH test just described or intramuscularly in the form of long-acting ACTH gel. Eight hours after ACTH administration is started, another total circulating eosinophil count is made. Normally, cortisone causes depression of eosinophil production. Therefore a normal cortisol response to ACTH stimulation would be a drop of total circulating eosinophils to less than 50% of baseline values. A drop of less than 50% is considered suspicious for adrenal insufficiency. False positive responses (less than a 50% drop) may occur in any condition that itself produces eosinophilia (e.g., acute episodes of allergy).

Adrenocorticotropic hormone (ACTH) assay. Plasma ACTH measurement has been used to help confirm the diagnosis of Addison’s disease and to differentiate primary from secondary adrenal failure. In primary adrenal failure, the ACTH value should be high and cortisol levels should be low. In hypothalamic or pituitary insufficiency, both ACTH and cortisol values theoretically should be low. Unfortunately, a considerable number of patients have cortisol or ACTH values within reference range. A specimen for plasma ACTH determination can be drawn at the same time as the specimen for baseline cortisol before stimulation tests and can be frozen for availability if needed.

Antiadrenal antibodies. In primary Addison’s disease, antiadrenal antibodies have been detected in 60%-70% of patients. This test would have to be performed in large reference laboratories or certain university medical centers. Currently, this test is not being used as a primary diagnostic procedure.

December 22, 2009
Primary Aldosteronism

Aldosterone is the major electrolyte-regulating steroid of the adrenal cortex. Production is stimulated by ACTH but is also influenced by serum sodium or potassium levels. In addition, aldosterone can be secreted under the influence of the renin-angiotensin system in quantities sufficient to maintain life even without ACTH. In plasma, some aldosterone is probably bound to alpha globulins. There is a circadian rhythm that corresponds to that of cortisol, with peak levels occurring in the early morning and low levels (50% or less of A.M. values) in the afternoon. Ninety percent of breakdown and inactivation takes place in the liver. Aldosterone acts primarily on the distal convoluted tubules of the kidney, where it promotes sodium reabsorption with compensatory excretion of potassium and hydrogen ions.

Primary aldosteronism (Conn’s syndrome) results from overproduction of aldosterone, usually by an adrenal cortex adenoma (carcinoma, nodular hyperplasia, and glucocorticoid-suppressible aldosteronism are rare etiologies). Symptoms include hypertension, weakness, and polyuria, but no edema.

Secondary aldosteronism refers to overproduction of aldosterone in certain nonadrenal conditions:

1. Hyponatremia or low salt intake
2. Potassium loading
3. Generalized edema (cirrhosis, nephrotic syndrome, congestive heart failure)
4. Malignant hypertension
5. Renal ischemia of any etiology (including renal artery stenosis)
6. Pregnancy or use of estrogen-containing medications

Current explanations for the effects of these conditions on aldosterone point toward decreased effective renal blood flow (Fig. 30-4). This triggers certain pressure-sensitive glomerular afferent arteriole cells called the “juxtaglomerular apparatus” into compensatory release of the enzyme renin. Renin acts on an alpha-2 globulin from the liver called “renin substrate” to produce a decapeptide, angiotensin I, which, in turn, is converted to an octapeptide, angiotensin II, by angiotensin-converting enzymes in lung and kidney. Angiotensin II has a very short half-life (< 1 minute), and is both a powerful vasoconstrictor and a stimulator of the adrenal cortex to release aldosterone. There are several feedback mechanisms, one of which is mediated by retention of salt (and accompanying water) by increased aldosterone, leading to increase in plasma volume, which, in turn, induces the juxtaglomerular apparatus of the kidney to decrease renin secretion directly. The autonomic nervous system also affects renin output. Normally, renin production follows a circadian rhythm that roughly (not exactly) parallels that of cortisol and aldosterone (highest values in early morning). There are reports that renin normal values diminish somewhat with age. Upright posture is a strong stimulus to renin secretion. Atrial natriuretic factor (a hormone produced in the atrial appendage of the heart that affects the kidney) acts as antagonist to aldosterone and to renin.

Fig. 30-4 Renal pressor system.

General laboratory findings

Hypokalemia is the most typical abnormality, and the combination of hypertension and hypokalemia in a person who is not taking diuretics suggests the possibility of primary aldosteronism. However, about 20% (range, 0%-66%) of patients with primary aldosteronism have serum potassium levels within population reference range. Most of these normokalemic patients have a serum potassium value that does not exceed 4.0 mEq/L (4 mmol/L). Also, hypokalemia in a person with hypertension who is taking diuretics does not mean that hypokalemia is invariably due only to the diuretic unless the patient had already been adequately investigated for Conn’s syndrome. Some other diseases that may be associated with both hypertension and hypokalemia include Cushing’s syndrome, essential hypertension combined with diuretic therapy, potassium-losing renal disease, licorice abuse, malignant hypertension, Bartter’s syndrome, and the 11-b-hydroxylase variant of congenital adrenal hyperplasia.

Other frequent laboratory findings in primary aldosteronism include mild alkalosis and a low urine specific gravity. Postprandial glucose tolerance (but not the fasting glucose) is abnormal in about 50% of patients. Hypernatremia is occasionally found in classic cases, but most patients have high-normal or normal serum sodium levels. Excretion of 17-OHCS and 17-KS is normal. Hemoglobin values and white blood cell counts are also normal.

Diagnostic tests

Serum potassium loading. Diagnosis of primary aldosteronism may be suggested by failure to raise serum potassium levels on regular diet plus 100 mEq of potassium/day. (In addition, urine potassium excretion should increase during potassium loading, assuming a normal salt intake of 5-8 gm/day.)

Plasma or urine aldosterone. Patients with primary aldosteronism secrete more aldosterone than normal persons, so that aldosterone levels are increased in 24-hour urine specimens and, under proper conditions, in plasma specimens. Although RIA methods are now being used, these are still too difficult for the average laboratory. Even reference laboratories are not always dependable, especially for plasma aldosterone assay. Urine assay has the advantage that short-term fluctuations are eliminated, and there appears to be less overlap between normal and abnormal values. Accurate collection of the 24-hour specimen is the major drawback. Plasma is much more convenient to obtain, but plasma levels are more likely to be affected by short-term influences and other factors. Among these are diurnal variation (lower values in the afternoon than in the morning) and upright position (which greatly increases plasma aldosterone), whereas a 24-hour urine collection dampens the effects of these changes and instead reflects integrated secretion rate. Both plasma and urine aldosterone levels are increased by low-sodium diets and by lowered body sodium content and are decreased in the presence of high sodium intake. A low serum magnesium level also stimulates production of aldosterone. Potassium has an opposite effect: high potassium levels stimulate aldosterone secretion, whereas hypokalemia inhibits it.

Since hypertensive patients are frequently treated by sodium restriction or diuretics that increase sodium excretion, and since the body may be sodium depleted while still maintaining serum sodium within a normal range, it is advisable to give patients a high-sodium diet or salt supplements for several days before collecting specimens for aldosterone (renin assay is invalidated by salt loading and would have to be done previously or at some other time). One investigator suggests adding 10-12 gm of salt/day for 5-7 days plus one additional day while a 24-hour urine for aldosterone is collected. Salt loading should not affect aldosterone values in primary aldosteronism, since the hormone is secreted autonomously and therefore production is not significantly affected by normal feedback mechanisms. As a further check, the 24-hour specimen (if urine is assayed) or a random urine specimen (if plasma is used) should be assayed for sodium. Urine sodium values less than 30 mEq/L (mmol/L) suggest decreased sodium excretion, implying a sodium deficit and therefore a falsely increased aldosterone level. Several investigators have used a saline infusion procedure (1.5-2.0 L of saline infused between 8 and 10 A.M., with plasma aldosterone samples drawn just before and after the infusion). Some data indicate that aldosterone normal values, like those of renin, decrease somewhat with age. Certain drugs may increase aldosterone secretion; most of these are agents that increase plasma renin (hydralazine [Apresoline], diazoxide [Hyperstat], nitroprusside, and various diuretics such as furosemide [Lasix]). Glucose ingestion temporarily lowers plasma aldosterone levels.

Plasma renin. Plasma renin characteristically is decreased in primary aldosteronism. Plasma renin assay is complicated by the fact that renin cannot be measured directly, even by RIA. Instead, angiotensin I generation is estimated. This situation is standard in laboratory measurement of enzymes: usually the action of the enzyme on a suitable substrate is measured rather than direct reaction of a chemical or antibody with the enzyme. Two related techniques are used: plasma renin activity (PRA) and plasma renin concentration (PRC). The PRA reflects the rate of angiotensin I formation per unit of time. However, renin substrate (on which renin acts) is normally not present in sufficient quantities for maximal renin effect. In PRC, excess renin substrate is added to demonstrate maximal renin effect; the result is compared with standard renin preparations to calculate the patient’s renin concentration. In most cases PRA is adequate for clinical purposes and is technically a little more easy. Estrogens increase renin substrate, so PRC would be more accurate in that circumstance.

Plasma renin is an immunoassay technique of moderate or moderately great difficulty; accurate results are highly dependent on the manner in which a laboratory performs the test. In addition, optimum conditions under which the assay should be performed are not yet standardized. Although several kits are now commercially available, these kits demonstrate considerable variation in results when tested on the same plasma samples. In short, most laboratories cannot be depended on to produce accurate results, and even reference laboratories may at times have problems, especially with some of the kits. Renin is a very unstable enzyme, and for usable results it must be drawn into an ethylenediamine tetraacetic acid (EDTA) hematology tube at room temperature and centrifuged at room temperature within 6 hours. After centrifugation, the plasma must be frozen in a nondefrosting freezer until assay. Heparin cannot be used. The tourniquet should be removed before the blood sample is drawn, since stasis may lower renin levels considerably. Certain factors influence renin secretion. Sodium depletion, hypokalemia, upright posture, various diuretics, estrogens, and vasodilating antihypertensive drugs (hydralazine, diazoxide, and nitroprusside) increase plasma renin levels. Methyldopa (Aldomet), guanethidine, levodopa, and propranolol decrease renin levels. Changes in body sodium have parallel effects on renin and aldosterone (e.g., low sodium level stimulates production of both), whereas changes in potassium have opposite effects (low potassium level stimulates renin but depresses aldosterone).

Although Conn’s syndrome is typically associated with low plasma renin levels, other conditions (Table 30-2) may produce similar decreased values. In addition, nearly 25% of hypertensive patients have decreased plasma renin levels without having Conn’s syndrome. In Conn’s syndrome, low-salt diet plus 2 hours of upright posture may increase previously low renin values but not enough to reach the normal range. In patients who do not have Conn’s syndrome, a renin level that is temporarily decreased for some reason will be stimulated enough to rise above the lower limits of normal. It has been advised not to use diuretics in addition to a low-salt diet plus upright posture, since the additional stimulation of renin production may overcome renin suppression in some patients with Conn’s syndrome. Four hours of continual upright posture was originally thought to be necessary; subsequent data indicated that 2 hours is sufficient.

Table 30-2 Typical renin-aldosterone patterns in various conditions

Several screening tests for renin abnormality have been suggested. The furosemide test seems to be the one most widely used.

Unilateral renal disease is another cause of hypertension that is potentially curable. It is estimated that 5% (or even more) of hypertensive persons have this condition, which makes it much more common than primary aldosteronism. Plasma renin is frequently elevated. Since renin assay may already be considered in a hypertensive patient to rule out Conn’s syndrome, it could also serve as a screening test for unilateral renal disease. However, about one half of patients (literature range, 4%-80%) who have curable hypertension due to unilateral renal disease have normal peripheral vein plasma renin levels.

Summary of tests in primary aldosteronism

Hypokalemia in a patient with hypertension should raise the question of possible Conn’s syndrome. The typical laboratory pattern for primary aldosteronism is that of increased aldosterone and decreased renin levels. Other conditions have symptoms or laboratory findings that might suggest primary aldosteronism, but these can be differentiated by the combination of aldosterone and renin (see Table 30-2). Some patients with Conn’s syndrome have aldosterone values in the upper normal area rather than definitely elevated values. At times, either the renin or the aldosterone assay will have to be repeated to obtain a correct diagnosis.

Adrenal localization procedures in aldosteronism

Primary aldosteronism actually comprises a group of at least three conditions. First is Conn’s syndrome, produced by unilateral adrenal tumor (adenoma, rarely carcinoma). This syndrome comprises about 70% (range, 54%-90%) of primary aldosteronism cases. Second is idiopathic aldosteronism, caused by bilateral nodular adrenal hyperplasia or associated with apparently normal adrenals. This category comprises about 25%–30% (range, 10%-45%) of primary aldosterone cases. Third is glucocorticoid-suppressible aldosteronism, in which elevated aldosterone levels are suppressible and treatable by cortisol or similar glucocorticoids. This condition is uncommon; is hereditary (autosomal dominant transmission); and is diagnosed by elevation of urine 18-oxocortisol (a metabolite of cortisol) greater than urine aldosterone.

Of these, Conn’s syndrome is responsible for about 75% of cases and is the only condition that is surgically curable. Several procedures have been advocated to help verify the diagnosis of Conn’s syndrome, to help differentiate it from the other categories of primary aldosteronism, and to establish which adrenal contains the adenoma. Currently, CT visualization of the adrenals is the most widely used technique. CT is reported to detect about 75% (60%-90%) of aldosterone-producing adenomas. Radionuclide adrenal scanning using one of several new radioactive agents concentrated by the adrenal is also useful in those few institutions that do the procedure. If CT or radionuclide scans are normal, some medical centers proceed to adrenal vein catheterization for aldosterone assay from the adrenal vein on each side. A considerable difference in concentration between the two sides suggests adenoma.

December 22, 2009
Cushing’s Syndrome. Part 2

48-hour dexamethasone suppression test.

The 48-hour DST is the most widely used confirmatory procedure. Dexamethasone (Decadron) is a synthetic steroid with cortisone-like actions but is approximately 30 times more potent than cortisone, so that amounts too small for laboratory measurement may be given to suppress pituitary ACTH production. The test is preceded by two consecutive 24-hour urine collections as a baseline. If low doses (2 mg/day) are used, patients with normal adrenal function usually have at least a 50% decrease (suppression) in their 24-hour urine 17-OHCS values compared to baseline, whereas those with Cushing’s syndrome from any etiology have little if any change. This test result is usually normal in those patients whose low-dose overnight DST is abnormal (nonsuppressed) only because of obesity. If larger doses (8 mg/day) are used, about 85% (range, 42%-98%) of those with adrenal cortex hyperplasia due to pituitary oversecretion of ACTH have at least a 50% decrease (suppression) of their 24-hour urine 17-OHCS values. Adrenal cortisol-producing adenomas or carcinoma rarely decrease their urine 17-OHCS levels. Patients with the ectopic ACTH syndrome due to bronchial or thymus carcinoids have been reported to produce false positive test results (decrease in urine 17-OHCS levels) in up to 40% of patients. Patients with the ectopic ACTH syndrome from lung small cell carcinoma or other tumors rarely change urine 17-OHCS levels. Since the test takes a total of 4 days (48 hours at baseline and 48 hours of test duration) and requires 24-hour urine collections, and since there are a significant number of exceptions to the general rules, plasma ACTH assay is supplementing or replacing the high-dose DST for differentiation of the various etiologies of Cushing’s syndrome. Some investigators report that the metyrapone test (discussed later) is better than the 48-hour high-dose DST in differentiating pituitary oversecretion of ACTH from adrenal tumor.

A single-dose overnight version of the high-dose DST has been reported, similar to the low-dose overnight test. A baseline serum cortisol specimen is drawn fasting at 8 A.M.; 8 mg of dexamethasone is given at 11 P.M.; and a second serum cortisol specimen is drawn fasting at 8 A.M. the next day. Normal persons and patients with pituitary ACTH syndrome have 50% or more cortisol decrease from baseline. Cortisol-producing adrenal tumors and ectopic ACTH patients have little change. Limited evaluation of this test reported similar results to the standard high-dose dexamethasone procedure.

Metyrapone test. Metyrapone (Metopirone) blocks conversion of compound S to cortisol. This normally induces the pituitary to secrete more ACTH to increase cortisol production. Although production of cortisol is decreased, the compound S level is increased as it accumulates proximal to the metyrapone block, and 17-OHCS or radioassay CPB methods for cortisol in either serum or urine demonstrate sharply increased apparent cortisol values (due to compound S) in normal persons and those with pituitary-induced adrenal cortex hyperplasia. Fluorescent assay or RIA for cortisol do not include compound S and therefore yield decreased cortisol values. Adrenal tumors are not significantly affected by metyrapone. Some authorities recommend measuring both cortisol and compound S. An increase in compound S verifies that lowering of the plasma cortisol level was accompanied by an increase in ACTH secretion. This maneuver also improves the ability of the test to indicate the status of pituitary reserve capacity, and the test is sometimes used for that purpose rather than investigation of Cushing’s disease. To obtain both measurements, one must select a test method for cortisol that does not also measure compound S. Compound S can be measured by a specific RIA method. Phenytoin or estrogen administration interferes with the metyrapone test.

Adrenocorticotropic hormone stimulation test. Injection of ACTH directly stimulates the adrenal cortex. Patients with cortex hyperplasia and some adenomas display increased plasma cortisol and 17-OHCS levels. If urine collection is used, a 24-hour specimen taken the day of ACTH administration should demonstrate a considerable increase from preinfusion baseline values, which persists in a 24-hour specimen collected the day after ACTH injection. Normal persons should have increased hormone excretion the day ACTH is given but should return to normal in the next 24 hours. Carcinoma is not affected. The ACTH stimulation test at present does not seem to be used very frequently.

Serum adrenocorticotropic hormone. Serum ACTH measurement by immunoassay is available in many reference laboratories. At present, the assay techniques are too difficult for the average laboratory to perform in a reliable fashion, and even reference laboratories still have problems with accuracy.

There is a diurnal variation in serum ACTH levels corresponding to cortisol secretion, with highest values at 8-10 A.M. and lowest values near midnight. Stress and other factors that affect cortisol diurnal variation may blunt or eliminate the ACTH diurnal variation. Serum ACTH data in adrenal disease are summarized in Table 30-1.

Table 30-1 Plasma adrenocorticotropic hormone in adrenal diseases

In Cushing’s syndrome due to adrenal tumor or micronodular hyperplasia, pituitary activity is suppressed by adrenal-produced cortisol, so the serum ACTH level is very low. In ectopic ACTH syndrome, the serum ACTH level is typically very high (4-5 times the upper preference limit) due to production of cross-reacting ACTH-like material by the tumor. However, some patients with the ectopic ACTH syndrome have serum levels that are not this high. In bilateral adrenal hyperplasia due to pituitary overactivity, serum ACTH levels can either be normal or mildly to moderately elevated (typically less than the degree of elevation associated with the ectopic ACTH syndrome). However, there is a substantial degree of overlap between pituitary tumor ACTH values and ectopic ACTH syndrome values. It has been suggested that ACTH specimens obtained at 10-12 P.M. provide better separation of normal from pituitary hypersecretion than do specimens drawn in the morning. Another study found that specimens drawn between 9:00 and 9:30 A.M. provided much better separation of normal from pituitary hypersecretion than specimens drawn at any other time in the morning. In summary, adrenal tumor (low ACTH levels) can usually be separated from pituitary-induced adrenal cortex hyperplasia (normal or increased ACTH levels) and from ectopic ACTH (increased ACTH levels). Pituitary-induced adrenal cortex hyperplasia has ACTH values that overlap with the upper range of normal persons and with the lower range of the ectopic ACTH syndrome. The time of day that the specimen is drawn may improve separation of normal persons from those with Cushing’s disease.

Corticotropin-releasing hormone test. About 85% of Cushing’s disease is due to pituitary hyperplasia or tumor, and about 15% is due to ectopic ACTH from a nonpituitary tumor. Corticotropin-releasing hormone (CRH) from the hypothalamus stimulates the pituitary to release ACTH (corticotropin). Ovine CRH is now available, and investigators have administered this hormone in attempts to differentiate adrenal tumor and the ectopic ACTH syndrome from pituitary overproduction of ACTH. Initial studies reported that after CRH administration, pituitary ACTH-producing tumors increased plasma cortisol levels at least 20% over baseline and increased their ACTH level at least 50% over baseline. Normal persons also increase their ACTH and plasma cortisol levels in response to CRH, and there is substantial overlap between normal response and pituitary tumor response. Primary adrenal tumors and the ectopic ACTH syndrome either did not increase cortisol levels or increased ACTH less than 50% and plasma cortisol levels less than 20%. However, several studies found that about 10% of pituitary ACTH-producing tumors failed to increase plasma ACTH or cortisol to expected levels. This is similar to the rate that pituitary tumors fail to suppress cortisol production as much as expected in the high-dose 48-hour DST. About 15% of patients with the ectopic ACTH syndrome overlap with pituitary ACTH tumors using the ACTH criteria already mentioned, and about 10% overlap using the plasma cortisol criteria. Therefore, differentiation of the etiologies of Cushing’s syndrome by the CRH test alone is not as clear-cut as theoretically would be expected. To prevent false results, the patients should not be under therapy for Cushing’s syndrome when the test is administered.

The CRH test has also been advocated to evaluate the status of pituitary function in patients on long-term, relatively high-dose corticosteroid therapy.

To summarize, the expected results from the CRH test after injection of CRH are (1) for a diagnosis of Cushing’s disease, an exaggerated response from adenoma of pituitary; (2) for Cushing’s syndrome of adrenal origin or ectopic ACTH syndrome, no significant increase in ACTH; (3) for the differential diagnosis of increased ACTH from pituitary microadenoma versus ectopic ACTH syndrome, inconsistent results. The CRH test is not completely reliable in differentiating primary pituitary disease from hypothalamic deficiency disease.

Cushing’s disease versus ectopic ACTH syndrome. The intracerebral inferior venous petrosal sinuses receive the venous blood from the pituitary containing pituitary-produced hormones; the right inferior petrosal sinus mostly from the right half of the pituitary and the left sinus from the left half. Several studies have suggested that catheterization of both inferior petrosal sinuses can differentiate ectopic ACTH production from the pituitary ACTH overproduction of Cushing’s disease in patients who do not show a pituitary tumor on computerized tomography (CT) scan or when the diagnosis is in question for other reasons. The most commonly used method is comparison of the ACTH level in the inferior petrosal sinuses with peripheral venous blood (IPS/P ratio) 3 minutes after pituitary stimulation by ovine CRH. Although several criteria have been proposed, it appears that an IPS/P ratio greater than 2.0 without CRH stimulation or a ratio of 3.3 or more in one of the inferior petrosal sinuses 3 minutes after CRH stimulation is over 95% sensitive and specific for Cushing’s disease versus ectopic ACTH syndrome (if technical problems are avoided). However, apparently this procedure is not as good in differentiating Cushing’s disease from pseudo-Cushing’s disease, since there is about 20% overlap with results from patients with some clinical or laboratory findings suggestive of Cushing’s disease (such as some patients with psychiatric depression) but without proof of pituitary hyperplasia or adenoma. In one study the same overlap was seen with clinically normal persons.

Computerized tomography

CT can frequently differentiate between unilateral adrenal enlargement (adrenal adenoma or carcinoma) and bilateral enlargement (pituitary hyperactivity or ectopic ACTH syndrome). However, it has been reported that nonfunctioning adrenal cortex nodules may occur in 1%-8% of normal persons, and one of these nodules could be present coincidentally with pituitary Cushing’s syndrome or ectopic ACTH. CT is very useful, better than pituitary sella x-ray films, in verifying the presence of a pituitary adenoma. Even so, third- and fourth-generation CT detects only about 45% (range, 30%-60%) of pituitary adenomas. In addition, it has been reported that 10%-25% of normal persons have a pituitary microadenoma, and some of these nonfunctioning nodules may be seen on CT and lead to a misdiagnosis of Cushing’s disease.

Summary of tests in Cushing’s syndrome

Currently, the most frequently utilized tests to screen for Cushing’s syndrome are the overnight low-dose DST and the test to detect abolishment of serum cortisol diurnal variation. Urine free-cortisol determination would provide more accurate information than the diurnal variation test. Confirmatory tests (if necessary) and tests to differentiate adrenal from nonadrenal etiology that are most often used are the 48-hour DST or the metyrapone test, serum ACTH assay, and CT visualization of the adrenals.

Conditions that affect the screening and confirmatory tests should be kept in mind. In particular, alcoholism (especially with recent drinking) and psychiatric depression can closely mimic the test results that suggest Cushing’s syndrome. Finally, there are some patients in each category of Cushing’s syndrome etiology who do not produce the theoretically expected response to screening or confirmatory tests.

December 22, 2009
Cushing’s Syndrome. Part 1

Cushing’s syndrome is caused by excessive body levels of adrenal glucocorticoids such as cortisol, either from (primary) adrenal cortex overproduction or from (secondary) therapeutic administration. This discussion will consider only the primary type due to excess adrenal production of cortisol. About 70% of cases (range 50%-80%) of Cushing’s syndrome due to adrenal overproduction of cortisol are caused by pituitary hypersecretion of ACTH leading to bilateral adrenal cortex hyperplasia. About 10% of cases are due to adrenal cortex adenoma, about 10% to adrenal cortex carcinoma, and about 10% to “ectopic” ACTH production by tumors outside the adrenal or pituitary glands, most commonly lung bronchial carcinoids (28%-38% of ectopic tumor cases) with the next most frequent being lung small cell carcinomas. A few cases are caused by thymus carcinoids, pancreatic islet cell tumors, pheochromocytomas, and various adenocarcinomas. One additional category is the uncommon syndrome of micronodular cortical hyperplasia, which biochemically behaves in a similar manner to adrenal cortex adenoma. Adrenal tumor is the most frequent etiology in patients younger than 10 years, and pituitary hyperactivity is the most common cause in patients older than 10. Cushing’s syndrome must be differentiated from Cushing’s disease, which is the category of Cushing’s syndrome due to pituitary hypersecretion of ACTH (usually due to a basophilic cell pituitary adenoma or microadenoma). The highest incidence of Cushing’s syndrome is found in adults, with women affected 4 times more often than men. Major symptoms and signs include puffy, obese-looking (“moon”) appearance of the face, body trunk obesity, “buffalo hump” fat deposit on the back of the neck, abdominal striae, osteoporosis, and a tendency to diabetes, hirsutism, easy bruising, and hypertension.

Standard test abnormalities

General laboratory findings include impairment of glucose tolerance in about 85% of patients (literature range, 57%-94%) that is severe enough to be classified as diabetes mellitus in about 25%. There is lymphocytopenia (usually mild) in about 80%, but most patients have an overall mild leukocytosis. Hemoglobin tends to be in the upper half of the reference range, with polycythemia in about 10% of affected persons. About 20%-25% have a mild hypokalemic alkalosis. The serum sodium level is usually normal but is slightly increased in about 5%. Total circulating eosinophils are usually decreased.

Screening tests

Urine 17-Ketosteroid assay. The urine 17-KS assay was one of the first tests used for diagnosis of Cushing’s syndrome. However, urine 17-KS values are increased in only about 50%-55% of patients with Cushing’s syndrome, and the test yields about 10% false positive results. Thus, 17-KS assay is no longer used to screen for Cushing’s syndrome. The 17-KS values may be useful in patients who are already known to have Cushing’s syndrome. About 45% of patients with adrenal adenoma and about 80%-85% (range 67%-91%) of patients with adrenal carcinoma have elevated urine 17-KS values. Patients with adrenal carcinoma tend to have higher urine 17-KS values than patients with Cushing’s syndrome from other etiologies, so that very high urine 17-KS values of adrenal origin suggest adrenal carcinoma.

Single-specimen serum cortisol assay. Laboratory diagnosis of Cushing’s syndrome requires proof of cortisol hypersecretion. For some time, assay of 17-OHCS in a 24-hour urine specimen or a single-specimen plasma 17-OHCS assay by the Porter-Silber method was the mainstay of diagnosis. However, in Cushing’s syndrome this technique yields about 15% false negative and 15% false positive results. The 17-OHCS values in urine are increased in some patients by obesity, acute alcoholism, or hyperthyroidism, whereas the 17-OHCS values in plasma are increased in many patients by stress, obesity, or an increase in cortisol-binding protein due to estrogen increase (oral contraceptive medication or pregnancy). Therefore, urine 17-OHCS assay and single determinations of plasma 17-OHCS are no longer considered reliable enough to screen for Cushing’s syndrome. Plasma or urine 17-OHCS assay was also used to measure adrenal response in stimulation or suppression tests. However, it has been replaced for this purpose by serum cortisol assay, which is technically easier to do and avoids the many problems of 24-hour urine specimen collections.

Single determinations of plasma cortisol, either in the morning or in the afternoon or evening, have the same disadvantages as plasma 17-OHCS and are not considered reliable for screening of Cushing’s syndrome. For example, single morning specimens detect about 65% of patients with Cushing’s syndrome (range, 40%-83%) and produce false positive results in about 30% of cases (range, 7%-60%). One report indicates that 11 P.M. or midnight specimens provide better separation of normal persons from those with Cushing’s syndrome.

Plasma cortisol diurnal variation. If plasma cortisol assay is available, a better screening test for Cushing’s syndrome than a single determination consists of assay of two plasma specimens, one drawn at 8 A.M. and the other at 8 P.M. Normally there is a diurnal variation in plasma levels (not urine levels), with the highest values found between 6 and 10 A.M. and the lowest near midnight. The evening specimen ordinarily is less than 50% of the morning value. In Cushing’s syndrome, diurnal variation is absent in about 90% of patients (literature range, 70%-100%). False positive results are obtained in about 20% of patients (range, 18%-25%). Therefore, significant alteration of the diurnal pattern is not specific for Cushing’s syndrome, since it is found occasionally in patients with a wide variety of conditions. Some of the conditions that may decrease or abolish the normal drop in the evening cortisol level in some persons are listed in the box on this page. Therefore, a normal result (normal circadian rhythm) is probably more significant than an abnormal result (although, as already noted normal plasma cortisol circadian rhythm may be present in about 10% of patients with Cushing’s syndrome).

Urine free cortisol. About 1% of plasma cortisol is excreted by the kidney in the original free or unconjugated state; the remainder appears in urine as conjugated metabolites. Original Porter-Silber chromogenic techniques could not measure free cortisol selectively. Fluorescent methods or immunoassay can quantitate free cortisol, either alone or with compound S, depending on the method. Immunoassay is becoming the most frequently used technique. Urine free-cortisol values in 24-hour collections are reported to be elevated in about 95% of patients with Cushing’s syndrome (literature range, 90%-100%) and to produce false positive elevation in about 6% of patients without Cushing’s syndrome (literature range, 0%-8%).

Urine free-cortisol levels may be elevated in some patients by some of the factors that affect blood cortisol, including severe stress, acute alcoholism, psychiatric depression, and occasionally patients with obesity. In cortisol-binding protein changes such as an increase produced by estrogens, most reports indicate that urine free-cortisol secretion levels are usually normal. Renal insufficiency may elevate plasma cortisol levels and decrease urine free-cortisol levels. Hepatic disease may increase plasma cortisol levels but usually does not affect urine free-cortisol levels significantly. The major difficulty with the test involves accurate collection of the 24-hour specimen. Also, the test is not performed in most ordinary laboratories and would have to be sent to a medical center or reference laboratory.

Some Conditions That Affect Serum Cortisol Diurnal Variation

Severe stress
Severe nonadrenal illness
Obesity
Psychiatric depression
Alcoholism (especially with recent intake)
Change in sleep habits
Encephalitis
Blindness
Certain medications (prolonged steroids, phenothiazines, reserpine, phenytoin, amphetamines)

Single-dose dexamethasone suppression test. The most simple reasonably accurate screening procedure is a rapid overnight dexamethasone suppression test (DST). Oral administration of 1 mg of dexamethasone at 11 P.M. suppresses pituitary ACTH production, so that the normal 8 A.M. peak of plasma cortisol fails to develop. After 11 P.M. dexamethasone, normal persons and the majority of obese persons have 8 A.M. plasma cortisol values less than 50% of baseline (predexamethasone) levels. Many endocrinologists require suppression to 5 µg/100 ml (138 nmol/L) or less. The consensus is that about 95% of Cushing’s syndrome patients exhibit abnormal test response (failure to suppress), although there is a range in the literature of 70%-98%). There is an average of less than 5% false positive results in normal control persons (range, 1%-10%).

There is controversy in the literature regarding certain aspects of this test. Some investigators found substantial numbers of patients with a Cushingoid type of obesity, but without demonstrable Cushing’s syndrome, who failed to suppress adequately (falsely suggesting Cushing’s syndrome) after the overnight DST. This involved 10% of Cushingoid obese patients in one series and 53% in another. Unfortunately, there are not many reports in the literature that differentiate lean from obese persons in control series. Another controversial point is the degree that the 8 A.M. cortisol specimen must be suppressed from baseline value to separate normal persons from those with Cushing’s syndrome. Some have found the standard of a 50% decrease from baseline values to be insufficiently sensitive, missing up to 30% of Cushing’s syndrome patients. These investigators suggest a fixed 8 A.M. plasma cortisol value (after dexamethasone) of 5 or 7 µg/100 ml. However, establishment of such a fixed value is complicated by the variations in cortisol reference ranges found in different methods and kits. Another problem are conditions that may produce false results (failure to suppress normally). Some of these are listed in the box on this page.

Phenytoin and phenobarbital affect cortisol by affecting the microsomal metabolic pathway of the liver. Estrogen increases cortisol-binding protein values, which, in turn, increases total plasma cortisol values. This may affect the DST when a fixed 5 µg/100 ml cutoff limit is used, since the already increased cortisol level must be suppressed even more than usual to reach that value. Spironolactone is a fluorescent compound and interferes with the Mattingly fluorescent assay technique. Immunoassay is not affected. Additional evidence to support abnormal screening test results may be obtained by using the standard DST.

Some Conditions That Interfere With the Low-Dose Overnight Dexamethasone Suppression Test

Conditions producing false normal test results*
Drug-induced interference (phenytoin, phenobarbital, estrogens, possibly spironolactone)
Conditions producing false abnormal test results†
Acute alcoholism
Psychiatric depression
Severe stress
Severe nonadrenal illness
Malnutrition
Obesity (some patients)
Renal failure
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* Apparent suppression of 8 A.M. cortisol in patients with Cushing’s syndrome.
† Failure to suppress 8 A.M. cortisol in patients without Cushing’s syndrome.

The single-dose DST and diurnal variation test may be combined. Plasma cortisol specimens are drawn at 8 A.M. and 8 P.M. Dexamethasone is administered at 11 P.M., followed by a plasma cortisol specimen at 8 A.M. the next day.

Confirmatory tests

Confirmation of the diagnosis depends mainly on tests that involve either stimulation or suppression of adrenal hormone production. It is often possible with the same tests to differentiate the various etiologies of primary hyperadrenalism. Normally, increased pituitary ACTH production increases adrenal corticosteroid release. Increased plasma corticosteroid levels normally inhibit pituitary release of ACTH and therefore suppress additional adrenal steroid production. Adrenal tumors, as a rule, produce their hormones without being much affected by suppression tests; on the other hand, they tend to give little response to stimulation, as though they behaved independently of the usual hormone control mechanism. Also, if urinary 17-KS values are markedly increased (more than twice normal), this strongly suggests carcinoma. However, hyperplasia, adenoma, and carcinoma values overlap, and 17-KS levels may be normal with any of the three etiologies.

December 22, 2009