Articles on Medical Diseases and Conditions

Month: December 2009

  • Phosphorus and Phosphate Abnormalities

    Phosphorus and phosphate are often spoken of interchangeably, although phosphorus is only one component of phosphate. The semantic problem is even more confusing because an order for “phosphate” assay usually results in laboratory measurement of inorganic phosphorus. However, much of the body phosphorus is a part of phosphate compounds. About 80%-85% of body phosphorus is found in bone and about 10% in skeletal muscle. Most body phosphorus is intracellular, where it represents the most abundant intracellular anion. Phosphorus is a part of phospholipid compounds in all cell membranes, adenosine triphosphate energy-transfer compounds, nucleic acids, the compound 2,3-diphosphoglyceric acid (which regulates oxygen affinity for hemoglobin), various enzymes, and the principal urinary acid-base buffer system. Phosphorus is acquired through food and absorbed through the small intestine. About 90% is extracted from serum by the kidney with about 85%-90% being normally reabsorbed by the renal proximal tubules. Serum phosphorus values change considerably during the day (variation of 2 mg/100 ml [0.65 mmol/L] within a reference range of 2.5-4.5 mg/100 ml [0.81-1.45 mmol/L]), with lowest values at 10-11 A.M. and highest at 10 P.M. -3 A.M. Therefore, values are usually higher in the late afternoon and evening than in the morning. Some of these changes are due to dietary factors and some to shifts between intracellular and extracellular localization. Phosphate excretion is low about 9 A.M.-1 P.M., high about 3 P.M. -8 P.M., low again about midnight-1 A.M., and high again about 3-5 A.M. Administration of glucose leads to a temporary shift of phosphorus from an extracellular to an intracellular location. If a glucose load is given orally, trough serum phosphorus values are found about 2 hours postprandially, and preingestion values are regained about 5 hours postprandially.

    Hypophosphatemia

    Most clinical abnormalities involving phosphorus are associated with hypophosphatemia. Symptoms include confusion, disorientation, delirium, and sometimes seizures, thus resembling the symptoms of hyponatremia and other metabolic encephalopathies. In addition, there is skeletal muscle weakness that may progress to actual myopathy. In chronic severe hypophosphatemia there may be bone abnormalities such as osteomalacia and pseudofractures, as well as hematologic abnormalities such as decrease in oxygen delivery by RBCs and a tendency toward hemolysis. WBC function may be disturbed, with an increased incidence of fungal and bacterial infection. Mild hypophosphatemia, on the other hand, is usually asymptomatic clinically and biochemically.

    The overall incidence of hypophosphatemia ranges from 2%-22% in hospitalized patients. The great majority of patients demonstrate only a mild degree of abnormality and no clinical effects. The box lists conditions more likely to be associated with severe hypophosphatemia. For example, severe hypophosphatemia may appear in chronically malnourished persons who undergo rapid refeeding with low-phosphate nutrients (nutritional recovery syndrome). In many of the conditions listed under severe phosphate deficiency, onset of hypophosphatemia may not appear until 1 or more days after onset of illness. The other conditions on the list frequently produce hypophosphatemia but usually only of moderate degree. Even more conditions may produce mild disorder. There is an association of hypophosphatemia with hypomagnesemia, especially in alcoholics.

    Selected Disorders Associated With Serum Phosphate Abnormality

    Phosphate decrease*
    Parenteral hyperalimentation
    Diabetic acidosis
    Alcohol withdrawal
    Severe metabolic or respiratory alkalosis
    Antacids that bind phosphorus
    Malnutrition with refeeding using low-phosphorus nutrients
    Renal tubule failure to reabsorb phosphate (Fanconi’s syndrome; congenital; vitamin D deficiency)
    Glucose administration
    Nasogastric suction
    Malabsorption
    Gram-negative sepsis
    Primary hyperthyroidism
    Chlorothiazide diuretics
    Therapy of acute severe asthma
    Acute respiratory failure with mechanical ventilation
    Phosphate excess
    Renal failure
    Severe muscle injury
    Phosphate-containing antacids
    Hypoparathyroidism
    Tumor lysis syndrome

    __________________________________________________________
    *Low phosphate diet can magnify effect of phosphorus-lowering disorders.

    In one study, the most common etiology was medication known to induce hypophosphatemia without phosphate supplements. This most often occurred in association with surgery. The second most common etiology was gram-negative sepsis.

    Hyperphosphatemia

    The most common cause of hyperphosphatemia is renal failure. Other causes are listed in the box on this page. Hyperlipidemia or RBC hemolysis may produce artifactual phosphate elevation. Hyperphosphatemia may lead to hypocalcemia.

  • Serum Magnesium Abnormalities

    Magnesium is the fourth most common body cation (after sodium, potassium, and calcium) and the second most common intracellular cation (after potassium). About half is located in soft tissue and muscle cells and about half is in bone. Only 1%-5% is extracellular. Most body magnesium is derived from food intake. About one third of dietary magnesium is absorbed, with the absorption site being the small intestine. Body magnesium is excreted by the kidney, primarily through glomerular filtration. Some tubular reabsorption also takes place. About 33% of serum magnesium (literature range, 15%-45%) is bound to serum proteins, 15% is complexed, and about 50% is free in the ionized form. Of the protein-bound fraction, about 75% is attached to albumin and most of the remainder to alpha-1 and alpha-2 globulin. Albumin thus carries about 30% (range, 25%-33%) of total serum magnesium. PTH is a very important regulator of magnesium blood levels through regulation of renal tubule reabsorption.

    Magnesium is important in protein synthesis, enzyme activation, and oxidative phosphorylation. It influences renal exchange of potassium and hydrogen ions and affects calcium levels. It also has an important role in nervous system control of muscle at the neuromuscular junction, where it slows neuromuscular impulse transmission by inhibiting acetylcholine. The major clinical symptoms of magnesium disorders are neuromuscular. Magnesium deficiency enhances muscle fiber excitability due to increased activity of acetyl-choline; this is manifested by muscle tremor, which can progress to seizures and tetany. Mental abnormalities include confusion, anxiety, and hallucination. Magnesium excess conversely displays antagonism of nerve impulse transmission and results in muscle weakness. Magnesium also exerts some effect on heart muscle. Decreased magnesium levels may produce or aggravate cardiac arrhythmias, whereas toxic levels of magnesium may be associated with heart block. Hypomagnesemia also potentiates the toxic effects of digitalis.

    Magnesium deficiency. Magnesium deficiency has been reported in about 10% (range, 7%-11%) of hospitalized patients. Some of the etiologies of hypomagnesemia are listed in the box. In addition, it has been reported that hypomagnesemia frequently accompanies hyponatremia (22%-27% of hyponatremic patients); hypocalcemia (22%-32% of patients); and hypophosphatemia (25%-29% of patients). Several studies also found that hypomagnesemia is frequent in patients with hypokalemia (38%-42%), but one study reported only 7%. Postoperative patients on IV feeding are reported to have a short-term, temporary 20% decrease in serum magnesium levels. Similar findings have been reported 12-24 hours after acute myocardial infarction, returning to previous levels by 48 hours, but not all studies agree.

    Excess magnesium. Increased serum magnesium levels are most often due to oliguric renal failure, which prevents excretion of magnesium. Overuse of magnesium compounds is an occasional etiology.

    Laboratory tests. RBCs contain 2-3 times the concentration of magnesium found in serum. Artifactual hemolysis thus may produce a significant increase in assay values. Skeletal muscle contains about 10 times the serum concentration. Since about 30% of serum magnesium is bound to albumin, and assays measure total magnesium levels, hypoalbuminemia will artifactually decrease serum magnesium levels.

    Magnesium Disorders
    Magnesium deficiency
    Alcoholism
    Malabsorption
    Malnutrition
    IV fluids without magnesium
    Severe diarrhea
    Diabetic ketoacidosis
    Hemodialysis
    Hypercalcemia
    Congestive heart failure
    Artifact (hypoalbuminemia)
    Certain medications
    Loop and thiazide diuretics
    Cyclosporine
    Cisplatin
    Gentamicin
    Magnesium excess
    Oliguric renal failure
    Overuse of magnesium-containing compounds
    Artifactual (specimen hemolysis)

    Various reports emphasize that serum magnesium values may not always truly reflect total body magnesium levels, since serum values may be falsely elevated in dehydration and falsely decreased in hemodilution with or without clinical edema or hypoalbuminemia. However, this problem is not unique to magnesium.

  • Hypocalcemia

    Hypocalcemia may be subdivided into nonionized hypocalcemia (decrease in serum total calcium value) and true hypocalcemia (decrease in ionized calcium value).

    Selected Etiologies of Hypocalcemia
    Artifactual
    Hypoalbuminemia
    Hemodilution
    Primary hypoparathyroidism
    Pseudohypoparathyroidism
    Vitamin D-related
    Vitamin D deficiency
    Malabsorption
    Renal failure
    Magnesium deficiency
    Sepsis
    Chronic alcoholism
    Tumor lysis syndrome
    Rhabdomyolysis
    Alkalosis (respiratory or metabolic)
    Acute pancreatitis
    Drug-induced hypocalcemia
    Large doses of magnesium sulfate
    Anticonvulsants
    Mithramycin
    Gentamicin
    Cimetidine

    The most common cause of nonionized (“laboratory”) hypocalcemia is a decrease in the serum albumin level, which lowers the total serum calcium value by decreasing the metabolically inactive bound fraction without changing the nonbound “ionized” metabolically active fraction. Therefore, this type of hypocalcemia is artifactual as far as the patient is concerned, since the metabolically active fraction is not affected. Sometimes nonionized hypocalcemia occurs with serum albumin values within the lower part of the population reference range, presumably because the previous albumin level was in the upper portion of the reference range. Although laboratory hypocalcemia is fairly common in hospitalized patients, true hypocalcemia is considerably less common than hypercalcemia. In one study, only 18% of patients with a decreased total serum calcium level had true hypocalcemia. Symptoms of decreased ionized calcium include neuromuscular irritability (Chvostek’s or Trousseau’s sign), which may progress to tetany in severe cases; mental changes (irritability, psychotic symptoms); and sometimes convulsions. Some causes of hypocalcemia are listed in the box on this page.

    Neonatal hypocalcemia. Neonatal serum calcium levels are lower than adult levels, with adult levels being attained at about 2 weeks of life for full-term infants and at about 4 weeks for premature infants. Neonates may develop hypocalcemia early (within the first 48 hours of life) or later (between age 4-30 days). Late-onset hypocalcemia can be due to a high-phosphate diet (cow’s milk), malabsorption, dietary vitamin D deficiency, alkalosis, and congenital disorders. The etiology of early-onset hypocalcemia is poorly understood. Symptoms include muscular twitching, tremor, and sometimes convulsions. Since one or more episodes of tremor or twitching are not uncommon in neonates, hypocalcemia is a rather frequent consideration in the newborn nursery. Conditions that predispose to early-onset neonatal hypocalcemia include maternal insulin-dependent diabetes, birth hypoxia, acidosis, respiratory distress, and low birth weight (usually associated with prematurity). There is a general inverse relationship between serum calcium level and birth weight or infant gestational age. Infants who are severely premature or have very low birth weight tend to develop hypocalcemia very early; in one study of such patients, one third became hypocalcemic by 15 hours after birth. In adult hypocalcemia, the diagnosis can be easily made with a serum total calcium assay if the patient has typical symptoms and if hypoalbuminemia is excluded. Ionized calcium assay may be necessary in equivocal cases. Although several formulas exist to predict ionized calcium using total calcium and serum albumin data, there is considerable disagreement in the literature whether these formulas are sufficiently accurate to be clinically useful. In one study on seriously ill adult patients, only about 20% of those who had formula-predicted ionized calcium deficit had measured ionized calcium abnormality. In newborns, serum calcium assay is much more difficult to interpret. First, neonatal calcium reference values increase with increasing gestational age, so that the reference range for prematures is different from the range for term infants. Second, there are surprisingly few data on neonatal reference ranges for calcium in the literature, and the data available are contradictory. For example, in laboratories with adult calcium reference range values of 9-11 mg/100 ml (2.25-2.75 mmol/L), the lower limit for premature infants varies in the literature from 6.0 to 8.0 mg/100 ml (1.50-2.0 mmol/L), and for full-term infants, from 7.3 to 9.4 mg/100 ml (1.83-2.35 mmol/L). If some other laboratory’s adult reference range were lower than 9-11 mg/100 ml, presumably the neonatal reference lower limit could be even lower than those quoted. High levels of bilirubin or hemoglobin (hemolysis) can affect (falsely decrease) several methodologies for serum calcium. Thus, laboratory results in possible early-onset neonatal hypocalcemia may be difficult to interpret.

    Laboratory tests

    Laboratory tests helpful in differential diagnosis are serum albumin, BUN, calcium, phosphorus, pH, and PCO2. These help to exclude hypoalbuminemia, chronic renal disease (BUN and phosphorus levels are elevated, pH is decreased), and alkalosis (respiratory or metabolic). Medication effect can be detected by a good patient history. Serum magnesium assay can exclude magnesium deficiency. If malabsorption is possible, serum carotene is a good screening test (Chapter 26). PTH assay is needed to diagnose hypoparathyroidism (PTH deficiency with decreased PTH levels) or pseudohypoparathyroidism (renal or skeletal nonresponse to PTH with increased PTH levels). N-terminal or “intact” PTH assay is better for this purpose than midregion or C-terminal assay if the patient has renal failure, since midregion and C-terminal fragments have a long half-life and thus accumulate in renal failure more than intact PTH or N-terminal fragments. If the BUN level is normal, there should be no difference between the various PTH assays.

    Vitamin D compound assay. Vitamin D is a fat-soluble steroid-related molecule that is absorbed in the small intestine. After absorption it is carried in chylomicrons or bound to an alpha-1 globulin called “transcalciferin.” Normally, about one third is metabolized to calcidiol (25-hydroxy-vitamin D) in the liver, and the remainder is stored in adipose tissue. Calcidiol is primarily regulated by the total amount of vitamin D in plasma from exogenous or endogenous sources; therefore, calcidiol is an indicator of vitamin D body reserves. Estrogen increases calcidiol formation. Calcidiol is altered to calcitriol (1,25-dihydroxy-vitamin D, the active form of vitamin D) in kidney proximal tubules by a 1-hydroxylase enzyme. Normal values decline with age. About 10% is metabolized to 24,25-dihydroxy-vitamin D by a different enzyme. As noted previously, calcitriol has actions affecting calcium availability in bone, kidney, and intestine. PTH and blood phosphate levels can influence the hydroxylase enzyme, with the effects of PTH being produced through its action on cyclic AMP.

    The vitamin D group includes two other compounds: Vitamin D2(ergocalciferol), derived from plant sources; and vitamin D3 (cholecalciferol), synthesized in the epidermis and therefore a naturally occurring form of vitamin D in humans.

    Laboratory assays for both calcidiol and calcitriol are available in some of the larger reference laboratories. These assays are useful mainly in patients with possible vitamin D overdose (hypervitaminosis D), in children with rickets, and in some adults with osteomalacia (the adult equivalent of rickets). Both osteomalacia and rickets are characterized by defective calcification of bone osteoid, and both involve some element of vitamin D deficiency.

    Vitamin D excess can produce hypercalcemia, hyperphosphatemia, soft tissue calcification, and renal failure. Calcidiol assay is the test of choice; the calcidiol level should be considerably elevated. In some patients with PHPT, the serum calcium level may be normal or borderline, and PTH assay may be equivocal. In these few patients, calcitriol assay may be useful, since it should be elevated in PHPT.

  • Serum Parathyroid Hormone-Related Protein (PTHrP)

    Since many patients (50% or more) with cancer and hypercalcemia do not have demonstrable bone metastases or PHPT, it has long been suspected that the cancer could be producing a parathyroid hormonelike substance. The parathyroid hormone-related protein (PTHrP) molecule has a C-terminal end and an N-terminal end like PTH; in addition, a portion of the PTHrP amino acid sequence is identical to that of PTH, although the majority of the PTHrP molecule is not. Also, it has been found that certain normal tissues can produce PTHrP (including the keratinized layer of skin epidermis, lactating breast tissue, placenta, adrenal, and a few others). PTHrP has recently been isolated and cloned, and antibodies have been obtained that react against it. Several investigators have reported results using homemade test kits, and one commercial kit is now available. Results thus far with these first-generation kits show that about 50% (range, 20%-91%) of patients with solid malignancies and hypercalcemia have increased PTHrP levels. Another 20% have bone metastases that could account for hypercalcemia without hormonal basis. It is currently thought that the other 30% may be producing some type of altered PTHrP that is not being detected by current antibodies. PTHrP assay may be useful when PTH assays fail to give expected results in patients with malignancy or give results that are borderline or slightly overlapping in nomogram areas between PHPT and tumor patients. However, PTHrP assays are not all alike and it is necessary to find a laboratory or kit that gives superior results.

  • Hypercalcemia and Malignancy

    In confirmed hypercalcemia, differential diagnosis is usually among PHPT, malignancy (metastatic to bone or the ectopic PTH syndrome), and all other etiologies. In most cases the differential eventually resolves into PHPT versus hypercalcemia of malignancy (HCM). There is no single laboratory test that can distinguish between PHPT and HCM every time with certainty. As noted previously, the better midmolecule PTH assays usually can differentiate normal from either PHPT or HCM and frequently can differentiate PHPT from HCM. If PHPT and HCM are not clearly separated, intact PTH assay might be obtained since it is generally better at separating PHPT and HCM. In any case a nomogram containing a scattergram of known cases is necessary. If the different PTH assays are not available, some other tests might indirectly provide evidence one way or the other. Hand x-rays are helpful if typical changes of PHPT are found (but this occurs in only a small percentage of cases). Renal stones are common in PHPT and uncommon in tumor. The quickest and easiest screening test for myeloma is serum protein electrophoresis, although serum and urine immunoelectrophoresis is more sensitive. A serum chloride value at the upper limit of the reference range or above is evidence against metastatic tumor. A bone scan and x-ray skeletal survey are useful to detect metastatic tumor. Some investigators advocate the assay of calcitonin, which is elevated with varying frequency in tumors associated with hypercalcemia and is usually not elevated in PHPT (some investigators report mild elevation in some patients). Unfortunately, regardless of the test results, PHPT may be present concurrently with malignancy in about 5% of patients with cancer.

    Serum calcitonin assay. Calcitonin (thyrocalcitonin, TCT) is secreted by nonfollicular C cells of the thyroid. An increased serum calcium level induces thyroid C cells to produce more calcitonin as part of hypercalcemia compensatory mechanisms. A major exception is PHPT, where the TCT level is usually normal or low, for poorly understood reasons (one report indicates an elevation in 10% of cases). The TCT level may be elevated in a considerable percentage of certain tumors known to metastasize to bone, such as lung carcinoma (about 30%-50% of cases; literature range 21%-62%) and breast carcinoma (about 50%; range, 38%-75%). Medullary thyroid carcinoma (MTC) produces elevated basal TCT in about 75% of cases (range, 33%-100%). Total serum calcium in MTC is usually normal. MTC or C-cell hyperplasia is found in >95% of patients with multiple endocrine neoplasia (MEN) syndromes type 2A and 2B. Type 2A also includes pheochromocytoma (about 50% cases) and PHPT (10%-25% cases). PHPT also is part of MEN type 1, which does not include MTC. The TCT level may be increased in the Zollinger-Ellison syndrome, as well as in certain nonneoplastic conditions such as chronic renal failure or pernicious anemia, and values may overlap with MCT. In summary, an elevated TCT level in a patient with possible PHPT raises the question of medullary carcinoma of the thyroid or some other malignancy, if the patient is not in renal failure.

    Ectopic parathyroid hormone syndrome.

    Nonparathyroid tumors that secrete PTH or PTH-like hormones (ectopic PTH syndrome) can produce considerable diagnostic problems. In one study, 19% of patients with tumor-associated hypercalcemia had no evidence of bone metastases. On the average, PTH assay values in ectopic PTH syndrome are lower than PTH values in PHPT. Although there is some overlap, the degree of overlap depends on the individual antiserum. There is disagreement regarding the nature of the ectopically produced hormone; that is, whether it is true PTH or a nonidentical molecule with a similar structure and PTH-like action that cross-reacts with most current PTH antisera. To further confuse matters, it is estimated that 5%-10% of patients with malignancy and hypercalcemia also will have a coexisting parathyroid adenoma with PHPT. It has also been stated that 15% of patients with PHPT have some coexisting disorder that could produce hypercalcemia.

  • Parathyroid Hormone (PTH)

    PTH is secreted in a discontinuous (pulsatile) fashion. There is a diurnal variation, with highest values at 2 A.M. (midnight to 4 A.M.) and lowest at noon (10 A.M. -2 P.M.. The parathyroids synthesize intact PTH, consisting of 84 amino acids in a single chain. Metabolic breakdown of intact PTH occurs both inside and outside of the parathyroids; outside the parathyroid, breakdown takes place in the liver and to a much lesser extent in the kidneys. This breakdown results in several fragment molecules: a small amino-terminal (N-terminal) fragment containing the PTH amino acid sequence 1-34; a larger midregion fragment containing amino acids 44-68; and a relatively large carboxy-terminal (C-terminal) fragment containing amino acids 53-84. Intact PTH and the N-terminal fragments have metabolic activity but not the midregion or C-terminal fragments. In a normal person, intact PTH constitutes 5%-15% of circulating PTH molecules. All PTH fragments are eliminated by the kidney, primarily through glomerular filtration. The measurable serum half-life of intact PTH is only about 5 minutes; that of the N-terminal fragment is about 2-3 minutes; and that of the C-terminal fragment is about 30 minutes. Renal function impairment will decrease elimination of the C-terminal fragment and also to a lesser extent the N-terminal fragment. In renal failure the C-terminal half-life lengthens to 24-36 hours and the N-terminal half-life lengthens to 30 minutes.

    PTH is measured by immunoassay. Original methods were based on antibodies against either the N-terminal fragment or the C-terminal fragment. Current tests use antibody against synthetic portions of the PTH chain, resulting in somewhat better sensitivity and reliability. These tests primarily detect either the C-terminal, the midregion fragment, or the intact PTH molecule. Actually, the C-terminal and midregion assays detect more fragments than the principal one indicated by their name:

    Assay
    Intact PTH
    N-terminal
    C-terminal
    Midregion (sometimes called “total” PTH)

    Assay includes
    Intact PTH only
    N-terminal fragment Intact PTH
    C-terminal fragment Intact PTH
    Midregion combined with C-terminal fragment
    Midregion fragment Intact PTH
    Midregion combined with C-terminal fragment

    At present, midregion assays have generally been more sensitive in detecting PHPT and separating PHPT from normal persons than C-terminal or intact PTH assays have been. Although there is considerable variation in reported sensitivity due to different kit antibodies and other technical factors, the best midregion kits are claimed to achieve 95% or greater sensitivity in detecting primary hyperparathyroidism. However, they are generally not as good in differentiating hypercalcemia due to PHPT from hypercalcemia of malignancy (the midregion PTH levels of 20%-25% of these cancer patients are normal or sometimes slightly increased rather than suppressed below reference range by the hypercalcemia). Intact PTH, on the other hand, generally is best at separating PHPT and hypercalcemia of malignancy (the PTH values of cancer patients are usually below intact PTH reference range or are in the lower part of the range, whereas the levels of PHPT patients are elevated or in the upper part of the range). The best intact assays are reported to detect PHPT almost as well as the better midmolecule assays. Intact PTH is also more reliable in patients with poor renal function. In azotemia, serum C-terminal and midregion fragments increase much more than intact PTH because of decreased excretion by the diseased kidneys.

    In some cases, detection of abnormality can be assisted by correlating PTH values with serum calcium values. PTH values may be within the upper part of the reference range but may still be higher than expected for the degree of serum calcium elevation. A PTH/calcium nomogram should be constructed for each PTH antiserum.

    Parathyroid hormone assay interpretation.

    Among diseases associated with hypercalcemia, PTH values are elevated in PHPT, in most cases of ectopic PTH syndrome, and in most cases of tertiary hyperparathyroidism. The actual percentage of elevated results in each category varies with the particular antiserum used (e.g., 8%-73% of PHPT patient values have been reported to be within the reference range with different anti-sera). In metastatic carcinoma to bone, the PTH value is normal with the majority of antisera, but there is overlap with PHPT in a significant minority of patients with nearly all antisera (the exact percentage varying with the particular antiserum). Parathyroid hormone values are usually normal or decreased in other conditions producing hypercalcemia.

    Parathyroid hormone values are elevated in many (but not all) conditions associated with true hypocalcemia (false hypocalcemia from hypoalbuminemia must be excluded). These include osteomalacia, vitamin D deficiency of dietary origin and in some patients with malabsorption, renal failure, and pseudohypoparathyroidism (congenital nonresponse of kidney to PTH). In PHPT, serum PTH and serum calcium levels are both increased.

    There are additional factors in PTH assay interpretation. PTH has a diurnal variation, with the lowest values (trough, nadir) about noon (10 A.M.-2 P.M.) and the peak about 2 A.M. (midnight-4 A.M.). Specimens should be drawn when patients are fasting at about 7-8 A.M. without using specimen anticoagulants. Specimens should be processed at cold temperatures, frozen immediately, and transported in dry ice. Assay of PTH at present is difficult. Reliable antisera are not yet readily available from commercial sources, and, as noted previously, homemade antisera in reference laboratories differ in reactivity.

    Problems and some solutions in PTH assay. Theoretically, any PTH assay should differentiate parathyroid tumor from various other etiologies of hypercalcemia, since PHPT should have increased PTH serum levels and hypercalcemia of all other etiologies should show decreased PTH secretion. Unfortunately, when tested with presently available antisera, some patients with PHPT may have PTH values within laboratory reference range (5%-73% in reports from different laboratories). In addition, some patients with hypercalcemia not due to PHPT may have values that are within the reference range rather than decreased. In most publications, results in hypercalcemia of malignancy fall within the reference range or below, but a few antisera permit some elevated values. In most reports there is substantial overlap between patients with PHPT and patients with hypercalcemia of malignancy when their values fall within the reference range, averaging about 10%-20% (literature range, 0%-73%). Diagnosis of parathyroid adenoma can be assisted by correlating PTH assay with serum calcium levels, based on the fact that the PTH level normally decreases as the serum calcium level increases. A parathyroid adenoma may produce a PTH level that is within population normal range but is higher than expected in relation to the degree of calcium elevation. A nomogram should be constructed for each PTH antiserum, correlating PTH values with serum calcium values obtained from patients with surgically proved PHPT. This nomogram also should provide data on PTH and calcium findings in other calcium-phosphorus disorders, such as metastatic carcinoma to bone, ectopic PTH syndrome, myeloma, and renal disease. The nomogram may permit separation of these conditions when it would be impossible with the numerical values alone. For example, in one report 45% of PHPT patients had PTH values within the PTH reference range; but with the use of the nomogram, 87% of the PHPT patients could be separated from normal persons. Therefore, with the majority of antisera, such a nomogram is almost essential when PTH values are interpreted, especially since results from different reference laboratories on the same patient specimens have shown considerable difference in behavior among different antisera when tested on patients with calcium disorders. These differences in results exist not only between C-terminal, midregion, and N-terminal categories of antisera but also between individual antisera within the same category. Use of a nomogram can reduce overlap between PHPT and malignancy to 5%-10% (literature range, 0%-15%).

    If the serum albumin level is low, the calcium (total calcium) level will be falsely decreased and could alter patient position in the nomogram. Correction of the calcium-albumin relationship by formula may help but may not be accurate. Also, the diagram block areas appear to clearly separate different categories of calcium disease, but, in fact there may be overlap between patient values in certain disorders, and the amount of overlap is different with each individual antiserum. To choose the best laboratory for PTH assay, I strongly suggest that each laboratory under consideration be required to supply a nomogram for each type of PTH assay they perform showing actual values from patients with proven calcium diseases, including hypercalcemia of malignancy, plotted on the diagram in the form of individual symbols, the symbols (dots, circles, triangles) representing the different diseases. It is necessary to have a substantial number of patient results in each disease category, especially in both PHPT and malignancy, to obtain an accurate picture. This way, it is possible to obtain a more meaningful comparison of actual PTH test results in different laboratories. The best PTH assay is one that not only clearly separates different diseases from the reference range but also has the least overlap between disease categories, especially in the area between PHPT and hypercalcemia of malignancy.

    If a patient has significant hypoalbuminemia, it may be better to ask for measurement of ionized calcium (which is not affected by the albumin level as total calcium is) and a PTH-calcium nomogram using ionized calcium and PTH values. The nomogram should have a scattergram of known PHPT and hypercalcemia of malignancy cases, not an empty block diagram only.

    Lesser used or historically important tests

    Tubular reabsorption of phosphate (phosphate reabsorption index). This procedure indirectly measures PTH by estimating PTH action on renal phosphate reabsorption. The patient should be on a normal phosphate (PO4) diet; a low-PO4 diet (<500 mg/day) raises tubular reabsorption of PO4 (TRP) normal values, whereas a high-PO4 diet (3,000 mg/day) lowers TRP normal values limits.

    The patient drinks several glasses of water and then voids completely. One hour after voiding, a blood sample is obtained for phosphorus and creatinine measurement. Exactly 2 hours after beginning the test, the patient again voids completely, and the urine volume and urine concentration of creatinine and phosphate are determined. It is then possible to calculate the creatinine clearance rate and find the amount of phosphorus filtered per minute by the glomeruli. Comparing this with the actual amount of phosphate excreted per minute gives the amount reabsorbed by the tubules per minute, or the TRP value. A rough approximation is afforded by the formula:

    %TRP = [1 – (UrinePO4 x serum creatinine /  Urine creatinine x serum PO4)]

    An index value of less than 80% means diminished TRP value and suggests PHPT. This test becomes increasingly unreliable in the presence of renal insufficiency. About 5% of patients with renal stones but without parathyroid tumor have TRP values of 70%-80%, whereas about 20% of patients with parathyroid tumors have normal TRP values. Therefore, a TRP reduction is more significant than a normal result. Hypercalcemia due to malignancy is usually associated with a decreased TRP value. In addition, some patients with other conditions such as sarcoidosis and myeloma have been reported to have reduced TRP values.

    X-ray findings. Bone changes highly suggestive of hyperparathyroidism may be found radiologically in about 15% of PHPT patients (literature range, 9%-36%), although the older literature reports some type of change in up to 46% of cases with skeletal surveys. The incidence of bone change has considerably decreased because of earlier diagnosis. The most typical findings are subperiosteal cortical bone resorption in the phalanges. Patients with chronic renal disease (secondary or tertiary hyperparathyroidism) may also demonstrate these abnormalities but do not have elevated serum calcium levels (except in tertiary hyperparathyroidism, in which case there should be obvious long-term renal failure). Serum alkaline phosphatase elevation in PHPT is highly correlated with the presence of bone changes. It would be unlikely to find skeletal changes in hand x-ray films if the serum alkaline phosphatase level is not elevated. Of course, the serum alkaline phosphatase level could be elevated for a variety of reasons in any individual patient with hypercalcemia.

    Serum chloride. Primary hyperparathyroidism tends to develop a hyperchloremic acidosis. Serum chloride is often elevated in PHPT (40%-50% of cases, if one excludes patients with conditions that lower serum chloride levels such as vomiting, diarrhea, or diuretic use). Less than 10% of patients with non-PHPT etiologies of hypercalcemia have elevated serum chloride levels. In one series, these were all patients with thyrotoxicosis or the ectopic PTH syndrome. A chloride/phosphorus ratio has also been proposed. This was found to be greater than 33 in about 94% of PHPT patients (without renal failure). However, results in other hypercalcemias have been variable, with the percentage of patients reported with a ratio greater than 33 having ranged from 4%-39%.

  • Tests Useful in Differential Diagnosis of Hypercalcemia

    Serum calcium. Routine serum calcium assay measures the total serum calcium value. Total serum calcium contains about 50% bound calcium (literature range, 35%-55%) and about 50% nonbound calcium (literature range, 35%-65%). (Traditionally, nonbound calcium was called “ionized” calcium and is also known as “free” or “dialyzable” calcium.) Bound calcium is subdivided into calcium bound to protein and calcium complexed to nonprotein compounds. About 45% of total calcium (30%-50%) is protein-bound, of which 70%-80% is bound to albumin. The remaining 5% (5%-15%) of total calcium is complexed to ions such as citrate, phosphate, sulfate, and bicarbonate, which are not part of the serum proteins. Ionized calcium levels can be measured directly by ion-selective electrode techniques or less accurately can be estimated from total serum calcium and albumin or total protein values using certain formulas. The most commonly used calcium correction formula is that of R.B. Payne:

    Adjusted calcium = (measured calcium – serum albumin) + 4.0

    with calcium in mg/100 ml and albumin in g/100 ml. In international system (SI) units, the formula reads:

    Adjusted calcium = (calcium – 0.025 albumin) + 1.0

    with calcium in mmol/L and albumin in g/L. Ionized calcium values are affected by serum pH (a decreased of 0.1 pH unit increases ionization by 1.5%-2.5%). If serum is exposed to air and stands too long, the pH slowly increases. There is a small diurnal variation in ionized calcium, with the peak most often about 9 P.M. and the nadir about 9 A.M. There also is a small diurnal variation in urine calcium, with the peak most often about 11 P.M. and the nadir about 11 A.M.

    Ionized calcium is not affected by changes in serum albumin concentration, which is a significant advantage over total calcium assay. A decrease in the serum albumin level by 1 gm/100 ml produces an approximate decrease in the (total) serum calcium level of approximately 0.8 mg/100 ml from previous levels (this is an average value, and could be more or less in any individual patient). Since a decrease in the serum albumin level is frequent in patients with severe acute or chronic illness, an artifactual decrease of the serum calcium level is likewise frequent in hospitalized patients. The ionized calcium value is regarded by many investigators as more sensitive and reliable than the total calcium value in detection of PHPT. A certain number of their PHPT patients had elevated ionized calcium levels but normal total calcium levels. However, some investigators do not find that ionized calcium materially assists detection or diagnosis of PHPT. Also, certain conditions that produce hypercalcemia, such as myeloma, sarcoidosis, hypervitaminosis D, and metastatic carcinoma to bone, may in some cases be associated with increased ionized calcium levels. Most laboratories do not have the equipment necessary to perform ionized calcium assay, and although there are formulas that estimate ionized calcium from total calcium plus serum protein levels, there is disagreement in the literature concerning which formula is best. There is also disagreement whether such estimates are reliable enough to be used in the diagnosis of PHPT. The consensus in the literature seems to be that ionized calcium may be helpful in the diagnosis of PHPT in a minority of patients, such as those with borderline total calcium values or those with hypoalbuminemia, and is best determined using ion-selective electrode methodology.

    Serum phosphate. Decreased serum phosphate is one of the classic biochemical findings in PHPT. Phosphate usually is measured as the phosphorus ion. Only about 10%-15% is protein bound. There is a diurnal rhythm, with higher values in the afternoon and evening, which may be as much as double those in the morning. Serum phosphate (phosphorus) has not proved as useful as the earliest studies suggested, since phosphate levels in PHPT that are below the population reference range tend to be limited to patients with more severe disease. In fact, the serum phosphate level is decreased in only about 40%-50% of PHPT cases (literature range, 22%-80%). The reference range is fairly wide, which can mask small decreases; and some conditions such as a high-phosphate or a low-calcium diet increase serum phosphate levels. Renal dysfunction severe enough to produce an elevated blood urea nitrogen (BUN) level raises the serum phosphate level. Various conditions other than PHPT can decrease serum phosphate levels (discussed later in this chapter). Among those associated with hypercalcemia and hypophosphatemia, besides PHPT, are some patients with malignancy and occasional patients with sarcoidosis, myeloma, hyperthyroidism, and vitamin D intoxication.

    Serum alkaline phosphatase. Alkaline phosphatase in nonneoplastic calcium disorders is an index of bone involvement. X-ray bone abnormalities in PHPT are reported in 23%-36% of patients, with most of the relatively few reports being in the older literature. X-ray studies of the fingers demonstrate the most typical changes. It is estimated that alkaline phosphatase elevation occurs in about 95% of patients with PHPT who have bone x-ray changes but in only about 10%-15% of those who do not (therefore, there would be an ALP level increase in 20% to 30% of all PHPT cases; however, some find x-ray changes in present-day PHPT in only 15% of cases). Metastatic malignancy can produce elevated alkaline phosphatase levels due to either bone or liver involvement.

    Urine calcium excretion. Excretion of calcium in urine is increased in about 75% of patients with PHPT (literature range, 50%-91%). In addition to PHPT, a considerable number of other conditions may produce hypercalciuria (e.g., idiopathic hypercalciuria, said to be present in nearly 5% of the population; bone immobilization syndrome; Cushing’s syndrome; milk-alkali syndrome; hypervitaminosis D; renal tubular acidosis; and sarcoidosis).

    Assay is performed on 24-hour urine specimens. There is disagreement in the literature on whether to collect the specimens with the patient on a normal diet, a normal diet minus milk, cheese, and other milk products, or a standard 200-mg low calcium diet. Most investigators and urologists seem to prefer a normal diet, at least for screening purposes. Reference ranges differ according to type of diet. The Sulkowitch test is a semiquantitative chemical procedure for urine calcium measurement that was widely used before 1960 but is rarely performed today.

    Urine phosphate excretion. The older literature states that phosphate excretion is increased in most patients with PHPT. There are surprisingly little data on this subject in recent literature. However, hyperphosphaturia probably is not as frequent today, just as decreased serum phosphate levels are seen much less frequently. Increased urine phosphate excretion in PHPT is expected in 70%-75% of cases. Phosphate depletion (due to prolonged vomiting, nasogastric suction, or ingestion of aluminum-type antacids) and chronic renal disease can reduce or eliminate phosphate hyperexcretion. Conditions that can produce increased urine phosphate excretion besides PHPT include renal triple phosphate lithiasis, osteomalacia, and, in some patients, hyperthyroidism, sarcoidosis, Cushing’s syndrome, and malignancy. Reference values depend on diet.

  • Primary Hyperparathyroidism (PHPT)

    PHPT is caused by overproduction or inappropriate production of PTH by the parathyroid gland. The most common cause is a single adenoma. The incidence of parathyroid carcinoma is listed in reviews as 2%-3%, although the actual percentage is probably less. About 15% (possibly more) of cases are due to parathyroid hyperplasia, which involves more than one parathyroid gland. The most frequent clinical manifestation is renal stones (see the box). The reported incidence of clinical manifestations varies widely, most likely depending on whether the patient group analyzed was gathered because of the symptoms, whether the group was detected because of routine serum calcium screening, or whether the group was mixed in relation to the method of detection.

    Symptoms or Clinical Syndromes Associated with Primary Hyperparathyroidism, with Estimated Incidence*

    Urologic: nephrolithiasis, 30%-40% (21%-81%); renal failure
    Skeletal: 15%-30% (6%-55%); osteoporosis, fracture, osteitis fibrosa cystica
    Gastrointestinal: peptic ulcer, 15% (9%-16%); pancreatitis, 3% (2%-4%)
    Neurologic: weakness, 25% (7%-42%); mental changes 25% (20%-33%)
    Hypertension: 30%-40% (18%-53%)
    Multiple endocrine neoplasia syndrome: 2% (1%-7%)
    Asymptomatic hypercalcemia: 45% (2%-47%)
    *Numbers in parentheses refer to literature range.

    About 5% (literature range, 2%-10%) of patients with renal stones have PHPT.

    Laboratory tests. Among nonbiochemical tests, the hemoglobin level is decreased in less than 10% of cases (2%-21%) without renal failure or bleeding peptic ulcer. A large number of biochemical tests have been advocated for diagnosis of PHPT. The classic findings on biochemical testing are elevated serum calcium, PTH, and alkaline phosphatase levels and a decreased serum phosphate level.

    Serum calcium (total serum calcium). Most investigators consider an elevated serum calcium level the most common and reliable standard biochemical test abnormality in PHPT. Even when the serum calcium level falls within the population reference range, it can usually be shown to be inappropriately elevated compared with other biochemical indices. However, PHPT may exist with serum calcium values remaining within the reference range; reported incidence is about 10% of PHPT patients (literature range, 0%-50%). Normocalcemic PHPT has been defined by some as PHPT with at least one normal serum calcium determination and by others as PHPT in which no calcium value exceeds the upper reference limit. Some of the confusion and many of the problems originate from the various factors that can alter serum calcium values in normal persons, as listed here.

    1. Reference range limits used. Reference range values may be derived from the literature or from the reagent manufacturer or may be established by the laboratory on the local population. These values may differ significantly. For example, the values supplied by the manufacturer of our calcium method are 8.7-10.8 mg/100 ml (2.17-2.69 mmol/L), whereas our values derived from local blood donors (corrected for effects of posture) are 8.7-10.2 mg/100 ml (2.17-2.54 mmol/L).
    2. The patient’s normal serum calcium value before developing PHPT compared with population reference values. If the predisease value was in the lower part of the population reference range, the value could substantially increase and still be in the upper part of the range.
    3. Diet. A high-calcium diet can increase serum calcium levels up to 0.5 mg/100 ml. A high-phosphate diet lowers serum calcium levels, reportedly even to the extent of producing a normal calcium value in PHPT.
    4. Posture. Changing from an upright to a recumbent posture decreases the serum calcium concentration by an average of 4% (literature range, 2%-7%). A decrease of 4% at the 10.5 mg/100 ml level is a decrease of 0.4 mg/100 ml. Therefore, reference ranges derived from outpatients are higher than those established in blood donors or others who are recumbent. This means that high-normal results for outpatients would appear elevated by inpatient standards.
    5. Tourniquet stasis. Prolonged stasis is reported to produce a small increase in serum calcium and total protein values.
    6. Changes in serum albumin concentration (discussed under ionized calcium).
    7. Laboratory error or, in borderline cases, usual laboratory test degree of variation.

    Malignancy-associated hypercalcemia (MAH)

    Malignancy may produce hypercalcemia in three ways. The first is primary bone tumor; the only common primary bone tumor associated with hypercalcemia is myeloma, which begins in the bone marrow rather than in bone itself. Hypercalcemia is found in about 30% of myeloma patients (literature range, 20%-50%). The alkaline phosphatase level is usually normal (reported increase, 0%-48%) unless a pathologic fracture develops. About 5% of acute lymphocytic leukemia patients develop hypercalcemia. The second cause of hypercalcemia in malignancy is tumor production of a hormone resembling PTH called parathyroid hormone-related protein. This is known as the “ectopic PTH syndrome,” sometimes called “pseudohyperparathyroidism” (about 50% of solid-tumor MAH). The most frequent source of solid-tumor MAH is lung carcinoma (25% of MAH cases) followed by breast (20%), squamous nonpulmonary (19%) and renal cell carcinoma (8%).

    The third cause of MAH is metastatic carcinoma to bone (about 20% of solid tumor MAH). The breast is the most frequent primary site, followed by lung and kidney. Although prostate carcinoma is frequent in males, prostatic bone lesions are usually osteoblastic rather than osteolytic and serum calcium is usually not elevated.

    In addition, in some studies about 5% of patients with hypercalcemia and cancer also had PHPT.

    Selected nonneoplastic causes of hypercalcemia Among the conditions traditionally associated with hypercalcemia is sarcoidosis. There seems to be a much lower incidence of hypercalcemia in these patients today than in the past. Estimated frequency of hypercalcemia in sarcoidosis is about 5%-10% (literature range, 1%-62%). Serum phosphate levels are usually normal. Many of these patients have increased urine calcium excretion; the exact percentage is difficult to determine from the literature. Bone lesions are reported in 5%- 16% of cases. Tertiary hyperparathyroidism is another cause of hypercalcemia. In chronic renal failure, secondary hyperparathyroidism develops, consisting of decreased serum calcium, elevated PTH, elevated serum phosphate, and elevated alkaline phosphatase levels and development of rental osteodystrophy. If renal failure persists for a long time, secondary hyperparathyroidism may become tertiary hyperparathyroidism, which displays elevated serum calcium, elevated PTH, decreased serum phosphate, and elevated alkaline phosphatase levels and bone lesions (i.e., most of the biochemical changes usually associated with PHPT, but with diffuse hyperplasia of the parathyroid glands rather than a single adenoma). Hyperthyroidism produces hypercalcemia in about 15% of thyrotoxic patients and alkaline phosphatase (ALP) elevation in about 40%. Lithium therapy frequently increases serum total calcium levels. Although most calcium values remain within population reference range, about 12% of patients on long-term lithium therapy become hypercalcemic and about 16% are reported to develop elevated PTH assay results. Thus, discovery of hypercalcemia becomes a problem of differential diagnosis, with the major categories being artifact, neoplasia, PHPT, and “other conditions.” The incidence of asymptomatic hypercalcemia in unselected populations subjected to biochemical test screening ranges from 0.1%-6%. Many of the diagnostic procedures for PHPT have been developed to separate PHPT from other possible causes of hypercalcemia.

  • Tests in Calcium Disorders: Hypercalcemia

    Symptoms referable to hypercalcemia itself are very nonspecific; they include vomiting, constipation, polydipsia and polyuria, and mental confusion. Coma may develop in severe cases. There may be renal stones or soft tissue calcification. Hypercalcemia is most often detected on routine multitest biochemical screening panels, either in asymptomatic persons or incidental to symptoms from some disease associated with hypercalcemia (see the box on this page). In asymptomatic persons, primary hyperparathyroidism (PHPT) accounts for about 60% of cases. In hospital admissions, however, malignancy is the etiology for 40%-50% of cases and PHPT accounts for about 15%.

    Regulation of Serum Calcium Levels

    Regulation of serum calcium levels is somewhat complex. The major control mechanism is parathyroid hormone (PTH). Normally, parathyroid secretion of PTH is regulated by a feedback mechanism involving the blood calcium level. A decreased serum calcium level induces increased secretion of PTH, whereas an acute increase of the

    Selected Etiologies of Hypercalcemia

    Relatively common

    Neoplasia (noncutaneous)
    Bone primary
    Myeloma
    Acute leukemia
    Nonbone solid tumors
    Breast
    Lung
    Squamous nonpulmonary
    Kidney
    Neoplasm secretion of parathyroid hormone-related protein (PTHrP, “ectopic PTH”)
    Primary hyperparathyroidism (PHPT)
    Thiazide diuretics
    Tertiary (renal) hyperparathyroidism
    Idiopathic
    Spurious (artifactual) hypercalcemia
    Dehydration
    Serum protein elevation
    Lab technical problem

    Relatively uncommon

    Neoplasia (less common tumors)
    Sarcoidosis
    Hyperthyroidism
    Immobilization (mostly seen in children and adolescents)
    Diuretic phase of acute renal tubular necrosis
    Vitamin D intoxication
    Milk-alkali syndrome
    Addison’s disease
    Lithium therapy
    Idiopathic hypercalcemia of infancy
    Acromegaly
    Theophylline toxicity

    serum calcium level decreases secretion of PTH. PTH has a direct action on bone, increasing bone resorption and release of bone calcium and phosphorus. In addition, PTH increases the activity of the activating enzyme cyclic adenosine monophosphate (AMP) in the proximal tubules of the kidney, which increases conversion of calcidiol (25-hydroxyvitamin D) to calcitriol (1,25-dihydroxy-vitamin D). Calcitriol has metabolic effects that help to increase serum calcium levels, such as increased renal reabsorption of calcium, increased GI tract absorption of calcium, and the drawing out of some calcium from bone. On the other hand, an increased calcitriol level also initiates a compensatory series of events that prevents the calcium-elevating system from overreacting. An increased calcitriol level inhibits renal tubule phosphate reabsorption, which results in loss of phosphorus into the urine. This leads to a decreased serum phosphate level, which, in turn, inhibits production of calcitriol. The actions of PTH, phosphate, and calcitriol produce a roughly reciprocal relationship between serum calcium and phosphate levels, with elevation of one corresponding to a decrease of the other. Both PTH (through cyclic AMP) and phosphate act on the same enzyme (25-OH-D 1 a-hydroxylase), which converts calcidiol to calcitriol.

    Besides PTH, a hormone called “calcitonin” has important, although subsidiary, effects on calcium metabolism. Calcitonin is produced in the thyroid gland, and secretion is at least partially regulated by serum calcium levels. Acute elevation of serum calcium leads to increased calcitonin secretion. Calcitonin inhibits bone resorption, which decreases withdrawal of calcium and phosphorus and produces a hypocalcemic and hypophosphatemic effect that opposes calcium-elevating mechanisms.

  • Serum Electrolyte Panels

    Many physicians order serum “electrolyte panels” or “profiles” that include sodium potassium, chloride, and bicarbonate (“CO2”; when “CO2” is included in a multitest electrolyte panel using serum, bicarbonate comprises most of what is being measured). In my experience, chloride and “CO2” are not cost effective as routine assays on electrolyte panels. If there is some necessity for serum chloride assay, as for calculation of the anion gap, it can be ordered when the need arises. Assay of serum bicarbonate is likewise questionable as a routine test. In most patients with abnormal serum bicarbonate values, there is acidosis or alkalosis that is evident or suspected from other clinical or laboratory findings (e.g., as severe emphysema or renal failure). In patients with acid-base problems, blood gas measurement or PCO2 measurement is more sensitive and informative than serum bicarbonate assay.