Each of the three categories of true laboratory thyrotoxicosis has a counterpart in which the apparent pattern does not reflect true thyroid hormone status. I would like to call the resulting test patterns “deceptive laboratory hyperthyroidism.” These patterns are misleading because of non thyroidal alteration of one or both thyroid hormone levels. Deceptive laboratory hyperthyroidism […]
The diagnosis of thyroid disease now depends as much on laboratory results as it does on clinical findings. One might therefore use the term “laboratory hyperthyroidism” when considering the spectrum of test results in thyrotoxicosis in the same manner that one employs the term “clinical hyperthyroidism” when evaluating patient signs and symptoms. Laboratory diagnosis is […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
