Articles on Medical Diseases and Conditions

Tag: Thyroid Tests

  • Comments Regarding Use of Thyroid Tests

    As previously noted, some thyroidologists and laboratorians advocate screening for thyroid disease with a single test, often citing need for cost containment. The ultra sensitive TSH appears to be advantageous for this purpose. The T4 test result is more frequently normal in mild disease and more frequently abnormal in the absence of thyroid disease than is the ultra sensitive TSH. The FT4 test has many of the same problems as the T4 test, although the FT4 is less frequently affected by non thyroid conditions. The major problem with the one-test approach is that the clinician becomes very dependent on the laboratory to select a reliable method or commercial kit. I can verify that not all commercially available kits are equally reliable and that it takes search for published evaluations of the exact manufacturer’s kit under consideration in addition to extensive evaluation of the kit in the potential user’s own laboratory to establish proof of reliability. In addition, each laboratory should establish its own reference range using a statistically satisfactory number (at least 20) of blood donors or other clinically normal persons.

    Finally, one has to consider the possibility of laboratory error, even though it may not be frequent on a statistical basis. For these and other reasons, some order two tests rather than one for screening purposes, such as the FT4 plus the ultra sensitive TSH. If both test results are normal, this is very reassuring. If one or both are abnormal, either or both can be repeated to verify abnormality. The diagnosis sometimes can be made with this evidence plus clinical findings, or additional tests may be needed.

    Since the T4 (or its variants) and the ultra sensitive TSH are being used extensively to screen for and diagnose thyroid disease, it might be useful to catalog the patterns encountered using these two tests and some of the conditions that produce these patterns (see the box).

  • Thyroid Tests in Hypothyroidism

    Serum thyroxine. Thyroxine is frequently used as the major screening test for hypothyroidism, since the T4 level is low in most cases. There is some overlap between hypothyroid patients and normal persons in the lower part of the T4 reference range, since persons with mild, early, or subclinical disease may be inadvertently included in groups of clinically normal persons used to establish the reference range. There is some evidence that nearly all hypothyroid patients within euthyroid population reference limits have T4 values in the lower 50% of the reference range, so that T4 values in the upper half of the reference range are generally reliable in excluding hypothyroidism. Laboratory error, of course, must be considered if the laboratory result does not conform to the clinical picture. If the patient specimens are kept at room temperature for more than 48-72 hours, as might happen when they are sent by mail, increase in fatty acids during transit may falsely increase T4values when competitive binding (displacement) T4 methods rather than radioimmunoassay methods are used. Conditions that alter T4 results, such as TBG changes, non thyroidal illness, and certain medications must be remembered. Some endocrinologists are using TSH assay as a screening test instead of T4.

    Triiodothyronine-radioimmunoassay. T3-RIA has not proved very useful in the diagnosis of hypothyroidism. The majority of reports indicate that one fourth to one third of hypothyroid patients have T3-RIA values within normal range. In some cases, typically in Hashimoto’s thyroiditis or after treatment of hyperthyroidism with radioactive iodine, it is thought that normal-range T3-RIA values are due to preferential secretion of T3 in what has been called the “failing gland syndrome.” Test alterations due to non thyroidal illness, age-related decrease, and TBG alterations further complicate interpretation.

    Thyroid hormone-binding ratio. The THBR (T3U) test is another test that has not been very helpful in screening for myxedema because of substantial overlap with the euthyroid reference range. The major benefit from its use in possible hypothyroidism is for detection of TBG abnormality.

    Serum thyrotropin (TSH) assay. Serum TSH levels are increased in the great majority of patients with primary hypothyroidism, and serum TSH assay is currently the most useful first-line confirmatory test. Since secondary hypothyroidism (pituitary failure) is uncommon and dysfunction due to hypothalamic etiology is rare, TSH assay has also been advocated as a screening test. Until recently TSH assay had not found wider use in screening for thyroid disease in general because of considerable overlap in the low range between hyperthyroid and euthyroid persons. This occurred because in most TSH assay kits the lower limit of the euthyroid reference range is relatively close to zero. In addition, these kits had relatively poor sensitivity in the low range, so that it was difficult to separate hyperthyroid values, which typically are subnormal, from zero on one hand and lower limit of normal on the other. Some euthyroid lowerormal specimens demonstrated the same problem. Therefore, TSH assay was restricted mostly to diagnosis of hypothyroidism. As mentioned earlier, several ultra sensitive TSH kits have recently become available that have adequate sensitivity in the low range to reliably separate decreased TSH values from low normal values. The ultra sensitive TSH is now being advocated by some investigators as the best single screening test for thyroid disease in general. But as I mentioned earlier, in my experience, at present all ultra sensitive TSH kits are not equally reliable. The TSH levels may be increased, usually (but not always) to mild degree, in some clinically euthyroid patients with a variety of conditions (see the box). When the TSH level is elevated in conditions other than primary hypothyroidism, TSH values are usually less than twice the upper reference range limit. However, sometimes they may be as high as 3 times the upper limit and occasionally even higher.

    Primary hypothyroidism constitutes 95% or more of hypothyroid cases. The TSH assay in conjunction with the serum T4 assay is sufficient for diagnosis in the great majority of these patients (decreased T4 level with TSH level elevated more than twice and preferably more than 3 times the upper reference limit). In equivocal cases, a TRH test may be useful either to confirm primary hypothyroidism or to differentiate primary from secondary or tertiary etiology. As noted in the section on the TRH test, usefulness of the TRH test may be limited in severe non thyroidal illness. In those circumstances, a TSH stimulation test might be useful.

    It has been reported that there are several subgroups of patients with primary hypothyroidism, ranging from those with classic symptoms and markedly elevated TSH values to those with milder symptoms and only mildly elevated TSH values to those with equivocal or single symptoms and T4 and TSH values remaining within population reference range and only the TRH test result abnormal.

    About 5%-10% of patients referred to psychiatrists with symptoms of mood depression (“melancholia”) have been reported to have laboratory evidence of hypothyroidism. This evidence ranges from decreased T4and elevated TSH levels to an exaggerated TRH test response as the only abnormality.

    In secondary hypothyroidism, the thyroid is normal but malfunction occurs in either the hypothalamus or the pituitary. Typically, both T4 (or FT4) and TSH values are decreased. In a few cases, the TSH value is within normal range; the TSH however is structurally defective and cannot stimulate the thyroid normally.

    Thyrotropin-releasing hormone (TRH) test. A more complete discussion of the TRH test is located in the early part of this chapter. The TRH test has been mentioned as a confirmatory test for hypothyroidism. The TRH test has also been used to differentiate secondary from tertiary hypothyroidism. A significant increase in TSH after administration of TRH should theoretically suggest a hypothalamic rather than pituitary etiology for nonprimary hypothyroidism. Unfortunately, 40% of TSH hyposecretors of pituitary origin demonstrate adequate response to TRH stimulation. Therefore, only proof of pituitary hyposecretion by a poor response is considered sufficiently reliable for definite diagnosis. Even a poor response may not be reliable in the presence of severe non thyroidal illness.

  • Thyroid Tests in Phenytoin Therapy

    There are conflicting statements in the literature on the effect of phenytoin (diphenylhydantoin; Dilantin) on certain thyroid tests. Dilantin is known to compete for binding sites on TBG, but it may also affect thyroid tests due to activation (“induction”) of liver microsomal enzymes, resulting in accelerated metabolic alteration of T4. The majority of reports indicate that T4, free T4, and T3-RIA values are somewhat decreased from pretreatment levels. The T4 level is said to decrease about 25%-33% from pretreatment level, whereas the T3-RIA value decreases about 15%. In one study, the T4 level decreased below reference range in 40% of patients, whereas the T3-RIA value remained within reference range in all patients. The THBR, TSH and TRH tests are not significantly affected.

  • Effects of Iodine on Thyroid Tests

    Iodine deficiency

    Iodine deficiency goiter is rare in the United States but still might be encountered by a physician. Iodine deficiency leads to an increase in RAIU. In mild or moderate iodine deficiency there is said to be a decrease in T4 and THBR values, but values usually remain within their reference range. Often T3-RIA is increased. Assay levels of TSH are usually normal. In severe iodine deficiency, T4 and THBR values often decrease and T3-RIA increases. The TSH level may be elevated but only to a mild or moderate degree. Thus, iodine deficiency may simulate thyrotoxicosis (goiter and RAIU increases), T3 toxicosis (goiter with normal or decreased T4 levels and elevated T3-RIA) or even hypothyroidism (decreased T4 level).

    Iodine excess

    As noted previously, excess iodine usually results from inorganic iodide, most commonly found in respiratory tract medications such as SSKI, or from organic iodine present in x-ray contrast media and in certain medications such as the antiarrhythmic drug amiodarone. With long-term SSKI therapy, when the medication is initially administered, there is a decrease in T4 values of varying degree accompanied by an increase in TSH values, followed by a return of the T4 and TSH values toward baseline levels in days or weeks. However, some of those whose T4 level stabilizes within reference range have persisting mildly elevated TSH level. A minority of patients have persistently decreased T4 and increased TSH values, which tend to be more frequent in patients with preexisting thyroid diseases such as Hashimoto’s thyroiditis, therapy with lithium carbonate, previous iodine deficiency, and patients with treated Graves’ disease or toxic nodules. On the other hand, inorganic iodide can occasionally induce thyrotoxicosis (Jod-Basedow disease), most commonly when administered to persons who are iodine deficient. As noted previously, certain x-ray contrast media (ipodate and iopanoic acid) may temporarily increase T4 levels (due to inhibition of T4 to T3 conversion) but usually only to a mild degree (20%-40% over baseline). This may or may not be accompanied by a mild increase in TSH values. Marked T4 elevation suggests Jod-Basedow disease. If this occurs, it is usually associated with a decreased RAIU value. Amiodarone is an antiarrythmia drug that contains a considerable amount of inorganic iodide. It produces more frequent and substantial thyroid test abnormalities than does x-ray contrast media. In some patients it may cause iodine-induced thyrotoxicosis (Jod-Basedow disease; more common in patients with preexisting iodine deficiency). In others, it may lead to iodine suppression of the thyroid (similar to SSKI therapy), producing decreased T4 and elevated TSH values. A third effect is inhibition of peripheral tissue conversion of T4 to T3; the patients remain euthyroid, but there is elevation of T4 and TSH in some patients, while T3-RIA is relatively normal. Overall, in patients on amiodarone therapy the T4 level is elevated over reference range upper limit in 20%-30% (range, 10%-40%) of patients; there are no clinical signs of hyperthyroidism, and the elevated T4 level is accompanied by increased reverse T3 and normal T3-RIA, THBR, and TSH values. The RAIU value can be decreased or normal. In those patients who develop iodine-induced clinically evident thyrotoxicosis, the T4, FT4, FT4 I, THBR, and T3-RIA values are elevated, whereas the TSH value is low. On the other hand, decreased T4 and elevated TSH values with clinical hypothyroidism occurs in 15%-20% of patients. In almost all patients with thyroid test abnormalities, the test results usually return to normal in 3-7 months after the end of amiodarone therapy.

  • Thyroid Function Tests: Thyroid scan

    Thyroid uptake of radioactive isotopes of iodine or technetium may be counted by a special radiation detector that produces a visual overall pattern of gland radioactivity. This permits visual localization of areas that may be hyperactive or hypoactive. Thyroid scan has two major applications:

    1. In patients with hyperthyroidism, the scan can differentiate between diffuse hyperplasia and a hyperfunctioning nodule (“toxic nodule”), entities that require different treatment. In addition, occasionally patients with symptoms of hyperthyroidism have chronic lymphocytic thyroiditis. Serum T4 and T3 levels are frequently elevated. Both elevated and low 24-hour RAIU values have been reported. Early (2-6 hours) RAIU values may be elevated more frequently than the 24-hour uptake. Thyroid scan frequently has a rather characteristic nonuniform appearance that can be helpful in suggesting the diagnosis. Factitious (self-medication) hyperthyroidism and subacute thyroiditis usually are associated with a low RAIU value in conjunction with an elevated T4 level (falsely low RAIU due to excess iodine or previous antithyroid therapy must be excluded).

    2. In patients with a palpable thyroid nodule, the scan may demonstrate lack of radioactivity in that area (“cold” nodule), suggesting a lack of functional activity that would increase suspicion of carcinoma. Most reports agree that a hyperfunctioning nodule is rarely malignant. Single nonfunctioning nodules have a 10%-20% incidence of malignancy. Truly nonfunctioning nodules are much more likely to be malignant than nodules that retain some function. At times, however, normal thyroid tissue above or below a nodule may contribute some degree of apparent function to a nonfunctioning area. However, there is some controversy over the usefulness of thyroid scanning in the evaluation of thyroid nodules for possible malignancy (see Chapter 33).

    Thyroid-stimulating immunoglobulin (TSI) assay

    A group of immunoglobulins (antibodies) of the IgG class that could prevent binding of TSH to TSH receptors on the thyroid cell membrane were discovered. The antibody also has the ability to stimulate adenylate cyclase to produce cyclic AMP production, which causes release of T4 and T3. The antibody can exert effect over a relatively prolonged period of time. The antibody was originally named “long-acting thyroid stimulator” (LATS) and was measured with a bioassay system (McKenzie) using mouse thyroid. The LATS was considered to be fairly specific for Graves’ disease, although it was also found in a few patients with Hashimoto’s disease and occasionally in low titer in some other conditions. It could be detected in only 40%-45% (range, 9%-55%) of patients with Graves’ disease. It was present more frequently when the patients had pretibial edema or hyperthyroidism with eye signs (but it was found in only about 15% of patients with so-called isolated Graves’ ophthalmopathy). The titer tended to decrease after 6-12 months of clinically active disease, after treatment was given, or when the disease became inactive. There was not a close relationship between titer and severity of illness.

    It was subsequently found that LATS would bind to human thyroid tissue but could not stimulate it, so that the antibody was renamed “mouse thyroid stimulator.” In addition, once serum containing LATS was incubated with human thyroid tissue, that serum no longer showed any LATS activity in the McKenzie mouse bioassay (i.e., the LATS activity was destroyed or neutralized).

    More recently, other related IgG antibodies were found that prevented adsorption of LATS onto human thyroid tissue and (as a group) was therefore called “LATS protector.” This abnormal IgG antibody (or antibodies) can be demonstrated by three different assay techniques. One technique used high-titer LATS antibody as the indicator system whose endpoint was to show that antibody in patient serum blocked uptake of the LATS antibody by human thyroid tissue; since the LATS antibody activity in the McKenzie bioassay would be neutralized by human thyroid tissue and the patient antibody prevented such neutralization, the patient antibody was called LATS protector antibody. A second technique measured the ability of patient antibody to inhibit or block the binding of radioactive TSH to human or animal thyroid acinar cell membranes. This has been called the “TSH-binding inhibiting immunoglobulin” (TBII) assay. The third technique measured the ability of the antibody to stimulate human or animal thyroid acinar cell membrane-bound adenylate cyclase, producing increased cyclic adenosine monophosphate (AMP) activity. The original assay method used human thyroid tissue, so the antibody was originally called “human thyroid-stimulating immunoglobulin.” Since animal thyroid tissue can be used, the assay now is simply called “thyroid-stimulating immunoglobulin” (TSI). For example, one modification of the TSI uses special TSH-dependent FRTL-5 rat tissue culture thyroid cells.

    The LATS-protector assay is reported to detect about 75%-80% (range, 60%-90%) of patients with Graves’ disease. The LATS-protector assay was very complicated; few laboratories performed the assay and few if any do now. The TBII technique is reported to detect about 70%-80% (range, 39%-100%) of Graves’ disease, and the TBI detects about 75%-80% (range, 18%-100%). Neither the TBII nor the TBI are simple or easy. Most laboratories using either technique use “homemade” reagents, which accounts for much of the great variation in sensitivity reported in the literature. Since many nonresearch laboratories base their test performance claims on data from one or more research laboratories using the same technique but their own reagents, performance claims based on someone else’s results may not be valid. Therefore, it is desirable to send specimens to laboratories that can verify better sensitivity based on adequate numbers of well-diagnosed patients with Graves’ disease that they assayed themselves. Besides sensitivity, it is necessary to question specificity, since different laboratories may find different numbers of false positive results when patients with hyperfunctioning thyroid nodules, nonfunctioning nodules or goiter, thyroiditis, autoimmune disorders, and clinically normal status are tested.

    At present, TSI assay is used mainly for patients with borderline or conflicting evidence of Graves’ disease, patients who have some condition that affects the results of other tests, or patients who have “isolated Graves’ ophthalmopathy” (a condition in which all standard thyroid tests are normal). However, a negative test result in most laboratories does not completely exclude the diagnosis, and there is a possibility of false positive results.

  • Thyroid Function Tests: Thyrotropin-releasing hormone (TRH) test

    Synthetic TRH (Thypinone) is now available. Intravenous bolus administration of TRH normally results in a marked rise in serum TSH levels by 30 minutes after the dose. Serum prolactin levels also increase. There is some disagreement as to how much TRH to administer, with doses reported in the literature ranging from 100-500 µg. Early studies reported that at least 400 µg was needed to obtain full TRH effect. Interpretation depends on whether the patient has evidence of hyperthyroidism or hypothyroidism. One problem, however, is disagreement in the literature concerning how much TSH increase over baseline is considered “exaggerated.” The normal limit of increase over baseline varies in the literature from 20-40 µU/ml, so that 25 µU/ml seems to be a reasonable compromise. Reactions to the TRH injection are uncommon, but can occur, and the patients should be closely monitored during the procedure. In general, the smaller the dose, the lower the incidence of reactions. Therefore, many laboratories and investigators use a 200-µg dose; and a few, even a 100-µg dose. However, I have not seen any reports that compared sensitivity of these doses to the gold standard of the 400- or 500-µg dose. Even if such a report appears, it would take several studies including a very substantial number of patients with hypothyroidism and hyperthyroidism to verify satisfactory performance.

    Drawbacks. (1) There is conflicting evidence in the literature regarding the effects of severe non thyroid illness on TRH test results. At least one report indicates that a blunted response may occur in some apparently euthyroid patients with depressed T4 levels associated with severe non thyroid illness and also in some patients with hypothyroidism who also had severe non thyroid illness. This would complicate the diagnosis of hypothyroidism in some cases as well as the differential diagnosis of primary versus secondary etiology. (2) Several reports suggest that TSH response to TRH may be less in elderly persons. (3) In addition, certain conditions such as psychiatric unipolar depression, fasting more than 48 hours duration, and therapy with aspirin, levodopa, or adrenocorticosteroids depress TSH response to TRH. (4) Patients should discontinue desiccated thyroid or T4 therapy for 3-4 weeks (literature range, 2-5 weeks) before a TRH test. (5) Another (although not major) drawback is the $30-$40 cost for TRH and the need for two or three TSH assay specimens.

    Thyrotropin-releasing hormone results in hyperthyroidism. In hyperthyroidism, pituitary TSH production is suppressed by direct effect of excess circulating T4/T3 on the pituitary, and TSH assay after TRH fails to demonstrate a significant degree of TSH increase from pretest baseline values (positive test result). Unfortunately, about 5% false positive results (failure to elevate serum TSH levels after TRH) have been reported in persons without demonstrable hyperthyroidism. A flat or blunted TSH response to TRH has also been reported in patients with autonomous thyroid nodules but no clinical evidence of hyperthyroidism, in a considerable number of patients after adequate treatment of Graves’ disease, and in some patients with multinodular goiter. Certain other conditions (discussed later) may also affect results. A flat TRH test result is therefore considered very suggestive but not conclusive evidence of thyrotoxicosis. A normal result (normal degree of TSH elevation after TRH) is considered very reliable in excluding thyrotoxicosis. For this reason the TRH test is currently considered the most reliable confirmatory procedure for hyperthyroidism and is the standard against which all other tests are compared for accuracy.

    Thyrotropin-releasing hormone results in hypothyroidism. In primary hypothyroidism the TRH test usually demonstrates an exaggerated TSH response. This may render the test useful in the occasional patient with both equivocal symptoms and equivocal serum TSH values. Theoretically, the TRH test should be able to differentiate between hypothyroidism from inability of the pituitary to secrete TSH due to pituitary disease (secondary hypothyroidism) and inability of the hypothalamus to secrete TRH (tertiary hypothyroidism). In pituitary disease, the serum TSH level should not rise significantly after TRH administration, whereas in hypothalamic disease there characteristically is a TSH response that is normal in degree but that is delayed for approximately 30 minutes. Unfortunately, a substantial number of patients with pituitary lesions demonstrate relatively normal or delayed TRH test response. Therefore, absent or markedly blunted response is strongly suggestive of primary pituitary disease, but response to TRH is not diagnostic of hypothalamic disease.

    Thyrotropin-releasing hormone results in psychiatric patients. There have been reports that the TRH test is useful in differentiating unipolar (primary depression only) from bipolar (manic-depressive) psychiatric illness and from secondary types of depression. In unipolar depression, TRH-induced TSH response is said to be blunted in up to two thirds of patients, whereas most patients with other categories of depression have normal TSH response. Occasional patients with symptoms of depression may actually have thyrotoxicosis (“apathetic hyperthyroidism”) and some may have hypothyroidism.

  • Thyroid Function Tests: Thyroid stimulation and suppression tests

    Thyrotropin stimulation test. In some patients with myxedema, the question arises as to whether the etiology is primary thyroid disease or a malfunction secondary to pituitary deficiency. Normally, administration of TSH more than doubles a baseline RAIU or T4 value. Failure of the thyroid to respond to TSH stimulation strongly suggests primary thyroid failure, whereas normal gland response implies either a pituitary or hypothalamic problem or else some artifactual abnormality in the original screening tests. The same procedure can be used to confirm the diagnosis of primary hypothyroidism, since the thyroid should not be able to respond to TSH. The test may be helpful in patients who have been on long-term thyroid hormone treatment and who must be reevaluated as to whether the original diagnosis of hypothyroidism was correct. The TSH stimulation test can be done while thyroid hormone is still being administered, whereas it would take several weeks after cessation of long-term therapy for the pituitary-thyroid relationship to reach pretherapy equilibrium. An occasional use for TSH stimulation is to see whether parts of the thyroid that are not functioning on a thyroid scan are capable of function (versus being not capable of function or being suppressed by hypersecretion from other thyroid areas).

    Drawbacks. The TSH stimulation test is performed using bovine TSH. Some persons form antibodies against this material that may interfere with future TSH assay or produce allergic reaction if TSH is used again. Therefore, the test is infrequently used today. To avoid this potential problem, some investigators use a T3 withdrawal test rather than TSH stimulation. A potential problem using RAIU in TSH stimulation tests is some correlation of patient iodine status to degree of RAIU response to TSH. In general, as iodine deprivation increases, RAIU response also increases; iodine overload decreases the RAIU response.

    Triiodothyronine withdrawal test. The patient is placed on T3 therapy for 1 month (instead of other therapy). The T3 is then discontinued for 10 days, after which a serum TSH assay is performed. With medication containing T4 it is necessary to wait at least 4 weeks after withdrawal before routine thyroid function tests (T4, T3, TSH), are performed to allow the thyroid-pituitary-hypothalamic feedback system to regain normal equilibrium. After T3 withdrawal, it takes only 10 days to achieve the same effect. If the patient has primary hypothyroidism, the serum TSH level will be elevated after 10 days without T3. In euthyroid persons or those with secondary and tertiary hypothyroidism, the TSH level will be normal or decreased. The major drawbacks to this procedure are the long time intervals necessary and the fact that not enough experience with this test has been reported to ascertain how many exceptions or false results may be expected. This test also is rarely used today.

    Thyroid suppression test. This is frequently called “T3 suppression,” although T4 could be used instead of T3, and the pituitary rather than the thyroid is the actual organ directly suppressed by T3 administration (the thyroid is affected secondarily). A standard dose of T3 is given daily for 1 week. In normal persons, exogenous T3 (added to the patient’s own T4) suppresses pituitary secretion of TSH, leading to a decrease in patient thyroid hormone manufacture. Values of RAIU or T4 after T3 administration drop to less than 50% of baseline. In hyperthyroidism, the thyroid is autonomous and continues to manufacture hormone (with little change in RAIU or T4 level), although the pituitary is no longer stimulating the thyroid. The suppression test is thought to be very reliable in confirming borderline hyperthyroidism, although there are reports that 25% or more of patients with nontoxic nodular goiter may fail to show suppression. The same basic technique may be used in conjunction with the thyroid scan to demonstrate that a nodule seen on original scan is autonomous. This may be helpful, since reports indicate that 50%-80% of toxic nodular goiter patients have normal RAIU values and many have normal T4 test results. The procedure must be used with caution in elderly persons or patients with cardiac disease. The T3 suppression test has been largely replaced by the TRH test.

  • Thyroid Function Tests: Serum thyrotropin assay (TSH)

    Thyrotropin previously was known as thyroid stimulating hormone (TSH), and the abbreviation TSH is still used. Direct assay of TSH is now possible with commercially available kits that are as easy to use as those for T4 assay. Thyrotropin has a diurnal variation of 2 to 3 times baseline (literature range, 33%-600%), with highest levels occurring at about 10-11 P.M. (range, 6 P.M.-2 A.M.) and lowest levels at about 10A.M. (range, 8 A.M.-4 P.M.).

    Results in thyroid disease. Serum TSH levels are elevated in about 95% of patients with myxedema due to primary thyroid disease, which comprises 95%-96% of hypothyroid patients. Serum TSH levels are low in most cases of secondary (pituitary or hypothalamic) myxedema (about 4% of hypothyroid patients). Some patients with secondary hypothyroidism have normal TSH values when one would expect low TSH levels. The pituitary of these patients is able to secrete a small amount of TSH, not enough to maintain normal T4 levels but enough to leave TSH values within reference range. The T3-RIA values in these patients may be decreased; but in some instances may be low normal, either because of preferential secretion of T3 rather than T4 by subnormal TSH stimulation or because of problems with kit sensitivity in low ranges. In primary hyperthyroidism, serum TSH is decreased; the percentage of patients with decreased values depends on the sensitivity of the particular kit used. Before 1985, most commercially available kits were poorly sensitive in the lower part of their range, and could not easily differentiate low values from zero or from lowormal values. Theoretically, in typical cases of hyperthyroidism the excess thyroid hormone produced causes the pituitary to completely stop production of TSH, reducing serum TSH to zero. The closer a TSH assay can approach zero in detecting TSH, the better it can differentiate between hyperthyroidism and certain other causes for decreased serum TSH (which usually produces a serum TSH value somewhere between lower reference limit and zero). Some nonhyperthyroid etiologies include partial pituitary insufficiency, early or mild hyperthyroidism in some patients, severe non thyroid illness in some patients (depending on the particular TSH assay kit), monitoring thyroid suppressive therapy, and dopamine or high-dose glucocorticoid therapy in some patients.

    Therefore, about 1985, many manufacturers had begun modifying their TSH kits so as to increase sensitivity at the lower end of the TSH assay range in order to differentiate lesser decreases of TSH (less likely due to hyperthyroidism) from marked decreases (more likely due to hyperthyroidism). Manufacturers called the new TSH kits “high-sensitivity,” “ultra sensitive,” or “first, second, and third generation.” Unfortunately, these terms were used in different ways, the two most common being the theoretical lower limit the assay might achieve under the best experimental conditions and the “functional” lower limit the assay usually did achieve with a between-assay coefficient of variation less than 20%. Based on the most common usage in the literature, the theoretical lower limit of detection for first-generation assays could detect TSH as low as 0.3-0.1 mU/L (µU/ml); but their functional lower limit usually was no better than 0.3, frequently was no better than 0.5, and sometimes was 0.6-1.0. Second-generation assays theoretically can detect between 0.1-0.01 mU/L. Most have a functional lower limit between 0.07-0.04 mU/L. Generally, a functional ability to detect less than 0.1 mU/L qualifies the assay for second-generation or high-sensitivity (ultra sensitive) status. In the 1990s, a few third generation kits have been reported; these have a theoretical lower limit of detection between 0.01-0.005 mU/L and functional detection at least below 0.01 mU/L.

    Drawbacks. Evaluations in the literature and my own experience have shown that not all TSH kits (either “standard” or ultra sensitive) perform equally well. Moreover, laboratory error may produce false normal or abnormal results. In addition, there are conditions other than hypothyroidism or hyperthyroidism that can increase TSH values or decrease TSH values (see box 1 and box 2).

    Conditions That Increase Serum Thyroid-Stimulating Hormone Values

    Lab error
    Primary hypothyroidism
    Synthroid therapy with insufficient dose; some patients
    Lithium or amiodarone; some patients
    Hashimoto’s thyroiditis in later stage; some patients
    Large doses of inorganic iodide (e.g., SSKI)
    Severe non thyroid illness in recovery phase; some patients
    Iodine deficiency (moderate or severe)
    Addison’s disease
    TSH specimen drawn in evening (peak of diurnal variation)
    Pituitary TSH-secreting tumor
    Therapy of hypothyroidism (3-6 weeks after beginning therapy [range, 1-8 weeks]; sometimes longer when pretherapy TSH is over 100 µU/ml); some patients
    Acute psychiatric illness; few patients
    Peripheral resistance to T4 syndrome; some patients
    Antibodies (e.g., HAMA) interfering with monoclonal sandwich method of TSH assay
    Telepaque (Iopanic acid) and Oragrafin (Ipodate) x-ray contrast media; some patients
    Amphetamines; some patients
    High altitudes; some patients

    Conditions That Decrease Serum Thyroid-Stimulating Hormone Values*

    Lab error
    T4/T3 toxicosis (diffuse or nodular etiology)
    Excessive therapy for hypothyroidism
    Active thyroiditis (subacute, painless, or early active Hashimoto’s disease); some patients
    Multinodular goiter containing areas of autonomy; some patients
    Severe non thyroid illness (esp. acute trauma, dopamine or glucocorticoid); some patients
    T3 toxicosis
    Pituitary insufficiency
    Cushing’s syndrome (and some patients on high-dose glucocorticoid)
    Jod-Basedow (iodine-induced) hyperthyroidism
    TSH drawn 2-4 hours after Synthroid dose; few patients
    Postpartem transient toxicosis
    Factitious hyerthyroidism
    Struma ovarii
    Radioimmunoassay, surgery, or antithyroid drug therapy for hyperthyroidism; some patients, 4-6 weeks (range, 2 weeks-2 years) after the treatment
    Interleukin-2 drugs (3%-6% of cases) or alpha-interferon therapy (1% of cases)
    Hyperemesis gravidarum
    Amiodarone therapy; some patients
    __________________________________________________
    *High sensitivity TSH method is assumed.

    Some cases of elevated free T4 levels accompanied by TSH values that were inappropriately elevated (above the lower third of the TSH reference range) rather than depressed have been reported. These cases have been due to TSH-producing pituitary tumors, defective pituitary response to thyroid hormone levels, or peripheral tissue resistance to the effects of thyroid hormone (laboratory error in T4 or TSH assay also must be excluded). Some (a minority) of the current TSH kits (predominantly the solid-phase double-antibody technique) may cross-react with hCG if hCG is present in very large amounts. These TSH kits will indicate varying degrees of (false) TSH increase during pregnancy and in the rare syndrome of hyperthyroidism due to massive hCG production by a hydatidiform mole or choriocarcinoma. Likewise, some of the double-antibody kits using the “sandwich” technique with one or both of the antibodies being the monoclonal type may become falsely elevated due to interference by a heterophil-type antibody in the patient serum that reacts with mouse-derived antibodies. This occurs because monoclonal antibodies are usually produced by a fused (hybrid) cell containing a mouse spleen cell that produces the antibody combined with a myeloma tumor cell that reproduces the hybrid for long periods of time. These human antimouse antibodies (HAMAs) usually occur without known cause and in one study were found in 9% of blood donors. Some manufacturers have attempted to counteract or neutralize the effect of these antibodies, with a variable degree of success. Another antibody problem of a different type concerns artifactual interference in TSH assay procedure by anti-TSH antibodies in patients who had previous TSH injections and who developed antibodies against the injected material.

    In severe non thyroid illness, it was soon noticed that a considerable number of the “ultra sensitive” TSH kits produced some abnormal results in clinically euthyroid patients. Previous “standard” TSH kits, although less sensitive in the lower range, generally produced normal TSH values in severe non thyroid illness. The percentage of abnormal results in the ultra sensitive kits varies between different kits. Most abnormality is reported in severe non thyroid illness, and is manifested by decreased TSH in about 15% of cases (range, 0%-72%) and elevated TSH in about 7% of cases (range, 0%-17%). When decreased in true hyperthyroidism, TSH usually is zero; whereas in non thyroid illness it is usually not that low (first generation TSH assays cannot make this distinction). Of those elevated, most (but not all) were less than twice the upper limit of the reference range. The decreased values return toward normal as the patient recovers; in some reports, most of the elevated values occurred in the recovery phase. This has been interpreted as pituitary inhibition during the acute phase of the illness and release of inhibition in the recovery phase

    There is some controversy whether or not elevated TSH levels in some of these conditions (except adrenal insufficiency and heterophil or anti-TSH antibodies) represent a mild hypothyroid state with the pituitary forced to secrete more TSH to maintain clinical euthyroidism. The serum TSH level in primary hypothyroidism is usually elevated to values more than twice normal and frequently to more than 3 times normal, whereas TSH levels in the other conditions frequently are less than twice normal and in most cases are less than 3 times normal.

  • Thyroid Function Tests: Total serum triiodothyronine (T3-RIA)

    Serum T3 may be assayed by the same technique as T4-RIA. Total serum T3 (T3-RIA) is a specific, direct measurement of T3 using anti-T3antibody and should not be confused with the test formerly called the T3U. As previously mentioned, the T3U (THBR) is primarily an estimate of serum protein unsaturated binding sites, which secondarily provides an indirect estimate of T4 but not of T3. It is unfortunate that many used to speak of the T3U as “the T3 test,” making it difficult to be certain whether T3U or T3-RIA was meant. Analogous to T4 assay, serum T3 consists mostly of protein-bound T3. Therefore, serum T3 measurement is affected by alterations in thyroxine-binding proteins in the same direction as serum T4, although to a slightly lesser degree than serum T4. One may obtain a free T3 index using T3-RIA and THBR (T3U) values, The T3-RIA may be elevated for 1-2 hours after T3 (liothyronine) administration. It may also be temporarily increased for several hours after desiccated thyroid intake

    Results in thyroid disease. Under usual circumstances, T3-RIA has at least as good sensitivity as T4 in detecting thyrotoxicosis. In fact, some investigators have stated that T3-RIA is the most sensitive test for standard hyperthyroidism associated with increase of both T4 and T3, occasionally demonstrating elevation at an early stage before T4 values have risen above reference range upper limits. In addition, T3-RIA helps to detect that form of hyperthyroidism known as “T3 toxicosis” in which the T3 level is elevated but not the T4 level. Triiodothyronine toxicosis has been estimated to comprise about 3%-4% of hyperthyroid patients.

    Drawbacks. Unfortunately, T3-RIA has several substantial drawbacks. First, although T3-RIA test kits are as easy to use as T4-RIA kits, there seems to be more variation in results among T3 kits from different manufacturers than among T4 kits. Second, as noted previously, T3-RIA is affected by thyroxine-binding protein alterations similarly to T4. Third, perhaps the most serious drawback is the strong tendency of many severe acute or chronic non thyroid illnesses to decrease T3-RIA values even though the patient remains clinically euthyroid. In many of these cases T4 conversion to T3 in peripheral tissues is temporarily decreased and instead is shunted toward reverse T3 production. The decrease associated with severe non thyroid illness varies but often is very substantial and is the most common cause for artifactual T3-RIA decrease. Therefore, this severely decreases the usefulness of the T3-RIA in hospitalized patients. Fourth, T3-RIA is not reliable in hypothyroidism, because there is considerable overlap between values from hypothyroid patients and the lowormal reference range. A few reports suggest that occasionally persons have mildly hypothyroid T4 levels but enough T3 secretion by the thyroid to maintain a clinically euthyroid state. Fifth, T3-RIA is increased in iodine deficiency.

    A sixth problem affecting T3-RIA is difficulty in defining the reference range. Persons over age 60 may have reference limits that are significantly lower than those for persons under age 60. Most studies have found a10%-30% decrease in mean values after age 60, although reports have varied from 0%-52%, possibly because the degree of age effect differs between individual manufacturer’s kits or there may have been some differences in the populations tested. Unfortunately, very few laboratories determine age-related values for the particular T3-RIA kit that they use. If the kit used by any individual laboratory is affected, this implies that since a result in an elderly person within the reference range but near the upper limit of the range could be artifactually decreased, that apparent normal value might in fact be elevated for that patient if the reference range was not age-corrected. A few studies report a lifelong decrease of 5-10 ng/ml/ 10 years. This implies that T3-RIA values in childhood are higher than those for adults. Finally, certain medications (e.g., propranolol, dexamethasone) have been reported to decrease T3-RIA levels, although not severely.

  • Thyroid Function Tests: Free thyroxine assay

    Another approach to the problem of thyroxine-binding protein alteration is to measure free T4 rather than total T4. The amount of protein-bound inactive T4 by itself has no direct influence on the serum level of metabolically active free hormone. The original Sterling technique involved separation of free from protein-bound T4 by a dialysis membrane after adding radioactive T4. The amount of free T4 was estimated indirectly by measuring total T4, obtaining the percentage of radioactivity in the dialysis fluid compared to total radioactivity added to the patient specimen measured before dialysis, and then calculating FT4 by multiplying the percentage of the dialysate radioactivity by total T4 quantity. This method generally gave normal results in patients with TBG abnormalities but frequently produced elevated results in patients with severe non thyroid illness. Several years later, Nelson and Tomei developed a modification of the dialysis method using a different dialysis solution buffer and measuring FT4 directly in the dialysis fluid using a more sensitive T4 immunoassay than was available to Sterling. Nelson’s results showed that most specimens were within reference range in both TBG abnormality and in severe non thyroid illness. Some investigators consider the Nelson equilibrium dialysis direct method to be the current FT4 gold standard. However, dialysis is time consuming, relatively expensive, and cannot be automated. Therefore, most laboratories use non dialysis immunoassay methods, which are commercially available based on several different principles but that are simple enough to be within the technical ability of most ordinary laboratories. The “two-step” FT4 is one such method; this involves tubes with anti-T4 antibody coating the tube walls. This antibody captures FT4 in patient serum but not T4 bound to serum proteins. The patient serum is then removed; the tube washed; and a solution containing T4 labeled with an isotope or an enzyme is added. The labeled T4 solution is removed after incubation. The amount of labeled T4 captured by the antibody on the tube surface is proportional to the amount of FT 4 in the patient sample (that is, how many antibody binding sites are occupied by patient FT4 and therefore not available to labeled T4). At present, most kit manufacturers use the “T4 analogue” method, because it is the easiest and least expensive. A synthetic molecule similar to T4 (T4 analogue) is created that will not bind to TBG but will compete with nonbound (free) T4 for anti-T4 antibody. This analogue is labeled with an isotope or enzyme system, so that the amount of analogue bound to the antibody is proportional to the amount of FT4 available. The analogue kits appear to function as well or slightly better than the FT4I in differentiating euthyroid persons from hyperthyroid and hypothyroid patients.

    Drawbacks. Unfortunately, in patients with severe non thyroid illness most of the first-generation analogue kits were falsely decreased as often as the ordinary T4 methods and more often than the FT4I. Although the reasons for this have been disputed, the consensus indicates that the analogues bind to albumin to some degree and also are affected by nonesterified fatty acids. Albumin is often decreased in severe non thyroid illness. The manufacturers now attempt to “correct”their analogue kits in various ways, most often by adding a blocking agent that is supposed to prevent analogue binding to albumin. At present, most analogue kits are less affected by non thyroid illness than previously, but they still are affected, with a rate of false decrease about the same as the FT4I. However, not all FT4 kits perform equally well. In several multikit evaluations, one-step analog kits gave decreased values in severe non thyroid illness in about 40% of patients (range, 2%-75%) and increased values in about 1% (range 0%-9%). In several different dialysis and several two-step method kits, there were decreased values in about 20% of patients (range, 0%-81%) and increased values in about 12% (range, 0%-42%). There was considerable variation in results between different kits. Heparin increases free fatty acid concentration, which falsely decreases some of the FT4 kit results, particularly some analog methods; ordinary total T4 is not affected. Some two-step FT4 kits can be affected, producing mildly elevated results in some cases.