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.

• hyperthyroidism
• nephrotic syndrome
• protein malnutrition
• renal failure

• acute hepatitis
• hypothyroidism
• hypothyroidism; primary
• hypothyroidism; secondary
• pregnancy
• estrogen administration

• chronic thyroiditis (Hashimoto's disease)
• drug-induced hypothyroidism
• Graves' disease
• subacute thyroiditis
• thyrotoxic periodic paralysis
• toxic nodular goiter

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