The T3RU test measures the level of proteins that carry thyroid hormone in the blood.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

Your health care provider will instruct you, if necessary, to stop taking drugs that may interfere with the test.

Drugs that can increase T3RU values include:

• Anabolic steroids
• Heparin
• Phenytoin
• Salicylates (high dose)
• Warfarin

The following can increase thyroxin binding globulin (TBG) levels:

• Androgens (male hormones)
• Serious illness
• Kidney disease

Drugs that can decrease T3RU values include:

• Antithyroid medications
• Birth control pills
• Clofibrate
• Estrogen
• Thiazides

Pregnancy can also decrease T3RU levels.

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.

This test is done as part of an evaluation of thyroid function. Your doctor may order this test if you have signs of a thyroid disorder. Thyroid function is complex and depends on the action of many different hormones, including thyroid-stimulating hormone (TSH), T3, and T4.

This test helps estimate the availability of thyroxin binding globulin (TBG). This is the protein that carries most of the T3 and T4 in the blood. The higher the level of TBG , the lower the value of T3RU. A higher T3RU value means less TBG is available, possibly as a result of hyperthyroidism.

Normal values range from 24 - 37%.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Greater-than-normal levels may indicate:

• Kidney failure
• Overactive thyroid (hyperthyroidism)
• Nephrotic syndrome
• Protein malnutrition

Lower-than-normal levels may indicate:

• Acute hepatitis
• Pregnancy
• Underactive thyroid (hypothyroidism, primary hypothyroidism, or secondary hypothyroidism)
• Use of estrogen

Abnormal results may also be due to an inherited condition of high TBG levels. Usually thyroid function is normal in these people.

Other conditions under which the test may be performed:

• Chronic thyroiditis (Hashimoto's disease)
• Drug-induced hypothyroidism
• Graves disease
• Subacute thyroiditis
• Thyrotoxic periodic paralysis
• Toxic nodular goiter

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism. Endocr Pract. 2002;8(6) 459.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Goldman: Cecil Medicine. 23rd ed. Philadelphia, Pa:Saunders; 2007:chap 244.