THYROID FUNCTION TEST

any clinical and laboratory procedure that assesses the efficiency of the thyroid gland in synthesizing the two most active thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Nonspecific thyroid tests measure the metabolic effects of the thyroid hormones (see basal metabolic rate); more specific tests measure one aspect or another of the synthesis, release, and transport of the thyroid hormones. The latter tests are generally based on the fact that thyroid tissue selectively concentrates iodine from the bloodstream, to incorporate it into the synthesis of its hormones. By administering radioactive iodine and following it at various steps in its metabolic pathway, thyroid activity can be determined. In the radioactive-iodine-uptake test, the proportion of ingested iodine that is accumulated by the thyroid gland is measured. The individual tested is usually given an oral dose of iodine-131 in capsule form, and the fraction of this that accumulates in the neck region, where the thyroid gland is located, is measured 24 hours later by a Geiger counter. Uptakes of less than 15 percent of the given dose tend to be associated with a deficiency of the thyroid gland. To detect possible hyperactivity of the thyroid gland, the thyroid-suppression test is useful. The test is based on the fact that the administration of a thyroid hormone will normally cause a decrease in the uptake of radioactive iodine by the thyroid gland (negative-feedback principle). When the thyroid is overactive and enlarged, however, as in Graves' disease, high uptake is maintained. If administration of 150 micrograms of triiodothyronine daily for a week fails to suppress uptake in any one subject, Graves' disease is usually present.