| Subject | Contents |
| Definition | A test that measures the RT3U in the blood. |
| Alternative Names | T3RU; Resin T3 uptake; T3 resin uptake; thyroid hormone-binding ratio |
| How the test is performed | Blood is drawn from a vein ( venipuncture ) or by capillary puncture. |
| How to prepare for the test | The health care provider may advise you to withhold drugs that may affect the test (see "special considerations"). |
| How the test will feel | When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. |
| Why the test is 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. |
| Normal Values | 24% to 37% |
| What abnormal results mean | Greater-than-normal levels may indicate:hyperthyroidismnephrotic syndromeprotein malnutritionrenal failure Lower-than-normal levels may indicate: acute hepatitishypothyroidismhypothyroidism; primaryhypothyroidism; secondarypregnancy estrogen administration Additional conditions under which the test may be performed:chronic thyroiditis (Hashimoto's disease)drug-induced hypothyroidismGraves' diseasesubacute thyroiditisthyrotoxic periodic paralysistoxic nodular goiter |
| What the risks are | The risk of the test is only that associated with venipuncture. |
| Special considerations | Drugs that can increase RT3U (that is, lower TBG) include anabolic steroids, heparin, phenytoin, salicylates (high dose), and warfarin. Drugs that can decrease RT3U (that is, raise TBG) include antithyroid agents, clofibrate, estrogen, oral contraceptives, and thiazides. |
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