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RT3U

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Contents of this page:

Alternative names   

T3RU; Resin T3 uptake; T3 resin uptake; Thyroid hormone-binding ratio

Definition    Return to top

This is a test that measures the RT3U in the blood.

How the test is performed    Return to top

Blood is drawn from a vein or by capillary puncture (finger prick).

How to prepare for the test    Return to top

The health care provider may advise you to withhold drugs that may affect the test (see "special considerations").

How the test will feel    Return to top

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    Return to top

This test is performed as part of a thyroid function test.

T4 (thyroxine) is the major hormone controlling the basal metabolic rate (metabolism). Most of the T4 in the blood is attached to proteins. These bound hormones serve as a T4 reservoir, because only free T4 is active in the body. The cells convert T4 to triiodothyronine (T3). Once converted, the T3 is involved in the synthesis of various proteins.

Measuring RT3U helps estimate the level of thyroxin binding globulin (TBG) -- the protein that carries most of the T3 and T4 in the blood. RT3U is inversely proportional to TBG levels: that is, if the TBG level goes up, the RT3U will go down.

Pregnancy, estrogen therapy, and oral contraceptives all can stimulate increased production of TBG by the liver. Androgens (male hormones), serious illness, and kidney disease can lower TBG. These changes in TBG will affect the level of free T4 and thus thyroid hormone functions. It is important to not be incorrectly diagnosed as having thyroid dysfunction, since it would result in inappropriate treatment.

Normal Values    Return to top

24% to 37%

What abnormal results mean    Return to top

Greater-than-normal levels may indicate:Lower-than-normal levels may indicate:Additional conditions under which the test may be performed:

What the risks are    Return to top

The risks of the test are only those associated with having blood drawn.

Special considerations    Return to top

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.

Update Date: 1/27/2004

Updated by: Todd T. Brown, M.D., Department of Endocrinology and Metabolism, Johns Hopkins Hospital, Baltimore, MD. Review provided by VeriMed Healthcare Network.

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