RT3U
Alternate Names : Resin T3 Uptake, T3 Resin Uptake, T3RU, Thyroid Hormone-Binding Ratio
Definition A test that measures the RT3U in the blood.
Why is the Test Performed?
This test is performed as part of a thyroid function test.
T4 (thyroxine) is the major hormone controlling the basal metabolic rate. Most of the thyroxine in the blood is bound to proteins; this acts as a reservoir of available thyroxine, because only free thyroxine is active on cells. Thyroxine appears to be converted to triiodothyronine (T3) within cells, before it enters the nucleus and interacts directly with DNA, eventually resulting in the production of various proteins by the cell.
RT3U indirectly quantitates thyroxin binding globulin (TBG), the protein that carries most of the T3 and thyroxine in the blood. RT3U is inversely proportional to TBG levels: if the TBG level goes up, the RT3U will go down. Pregnancy, estrogen therapy, or oral contraceptives, for example, may stimulate increased production of TBG by the liver. Androgens, serious illness, and renal disease may lower TBG. These changes in TBG will affect the level of free thyroxine and thus thyroid hormone functions. It is important to not be diagnosed as having thyroid dysfunction, since it would result in inappropriate treatment.
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