RT3U

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

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

How the test is performed
Blood is drawn from a vein or by capillary puncture (finger prick).

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 (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

24% to 37%

What abnormal results mean

Greater-than-normal levels may indicate:

• 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.

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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