The thyroid uptake test is ordinarily used in conjunction with an immunoassay for total T4 to correct for the influence which alterations in the levels of circulating thyroid hormone-binding proteins have on the total T4 level. These proteins reversibly bind both T4 and T3, but have an affinity up to six times greater for T4. It is the unbound or free hormones that are believed to be physiologically active.
A patient's Percent Uptake (%TU) level is determined both by the concentration of thyroid hormone-binding proteins, principally TBG, and by the degree to which these proteins are saturated with thyroid hormones, principally T4. Increased hormone production by the thyroid gland will cause an increase in the saturation of TBG by T4, which will be reflected in a decreased Percent Uptake level. Similarly, decreased hormone production will be reflected in an increased Percent Uptake level.
The impression of hyperthyroidism, hypothyroidism or euthyroidism — created by a T4 result that is (respectively) elevated, depressed or within euthyroid reference limits — is confirmed if the thyroid uptake result is likewise elevated, depressed or within euthyroid reference limits. Where the two tests disagree, it is desirable to calculate a Free T4 Index (FT4I) from the total T4 and Percent Uptake results, to determine how far alterations in circulating thyroid hormone carrier proteins.
A high T4 level accompanied by a low Percent Uptake level suggests an increase in the concentration of thyroid hormone-binding proteins. This situation typically occurs in women who are either pregnant or on oral contraceptives, because of the increase in TBG levels caused by rising estrogen levels. The situation can also occur in cases of feminizing tumors, hyperproteinemia, acute intermittent porphyria, acute liver disease and hereditary TBG increase.
A low T4 level accompanied by a high Percent Uptake level suggests a lower than normal concentration of thyroid hormone-binding proteins. Such a condition can be caused by an elevated level of androgens (glucocorticoids, androgenic steroids, synthetic androgens, virilizing tumors, acromegaly), by hypoproteinemia (nephrotic renal disease, chronic debilitating disease), or by stress (surgical trauma, acute infection, supraventricular tachycardia, congestive heart failure). It can also be caused by the administration of certain drugs, for example, hydantoins or large doses of salicylates.
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