RT3U

Definition

The RT3U test measures the level of thyroid hormone-binding proteins in the blood.

How the test is performed

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test

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

For infants and children:

The preparation you can provide for this test depends on your child's age and experience. For specific information regarding how you can prepare your child, see the following topics:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation  (12 to 18 years)

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.

The thyroid hormones T3 and T4 are transported in the body via proteins that bind them. Only free (unbound) T3 and T4 are active in the body. Measuring T3 uptake (the RT3U test) helps estimate the availability 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 values will go down. A higher RT3U value means less TBG is available, possibly as a result of hyperthyroidism.

Pregnancy, estrogen therapy, and oral contraceptives can stimulate increased production of TBG by the liver. Androgens (male hormones), serious illness, and kidney disease can lower TBG levels. These changes in TBG will affect the level of free T4 and thus thyroid hormone functions. It is important to avoid incorrect diagnosis of thyroid dysfunction, since this 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

What the risks are

Risks associated with having blood drawn are slight:

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Special considerations

Drugs that can increase RT3U values (that is, lower TBG) include anabolic steroids, heparin, phenytoin, salicylates (high dose), and warfarin.

Drugs that can decrease RT3U values (that is, raise TBG) include antithyroid agents, clofibrate, estrogen, oral contraceptives, and thiazides.

Page Content:

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