Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism.

A laboratory test can be done to measure the amount of T3 in your blood.

See also:

• T4 test
• TSH test

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 T3 measurements include:

• Birth control pills
• Clofibrate
• Estrogens
• Methadone

Drugs that can decrease T3 measurements include:

• Anabolic steroids
• Androgens
• Antithyroid drugs (for example, propylthiouracil)
• Lithium
• Phenytoin
• Propranolol

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) and T4.

Sometimes it can be useful to measure both T3 and T4 when looking at thyroid function. For example, in some cases of hyperthyroidism, T3 may be increased but T4 may be normal.

The T3 test can measure both the T3 that is attached to proteins and floating free in the blood.

The range for normal values is 100 to 200 ng/dL (nanograms per deciliter).

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:

• High levels of a protein that carries T3 in the blood (may occur in pregnancy, with use of birth control pills or estrogen, liver disease, or as part of an inherited condition)
• Hyperthyroidism (for example, Graves' disease)
• T3 thyrotoxicosis (rare)
• Thyroid cancer (rare)

Lower-than-normal levels may be due to:

• Long-term illness
• Hypothyroidism (for example, Hashimoto's disease)
• Starvation

Other conditions under which the test may be performed:

• Painless (silent) 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)

Pregnancy and liver disease will falsely raise the T3 level. In these cases, it is useful to measure either the free T3 level or to perform the T3RU test.

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.