Definition of T3:

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

How the test is performed:

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.

How to prepare for the test:

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

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 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.