triiodothyronine resin uptake test

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a thyroid hormone that contains iodine and is liberated from thyroglobulin by hydrolysis. It has several times the biologic activity of thyroxine.
free triiodothyronine the fraction of triiodothyronine in the serum that is not bound to a binding protein.
triiodothyronine resin uptake test a thyroid function test, measuring how many sites on thyroxine-binding globulin (TBG) are occupied by endogenous triiodothyronine (T3) and how many sites remain available. An excess of radioactive exogenous triiodothyronine is added to the sample, followed by the addition of a resin that also binds T3. A portion of the radioactive T3 binds to sites on TBG not already occupied by endogenous thyroid hormones, and the remainder binds to the resin. The amount of labeled hormones bound to the resin (the triiodothyronine resin uptake) can be subtracted from the total that was added and the remainder is the amount that bound to the unoccupied binding sites on the thyroxine-binding globulin.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.