Triiodothyronine; T3 radioimmunoassay; Toxic nodular goiter - T3; Thyroiditis - T3; Thyrotoxicosis - T3; Graves disease - T3
Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism (the many processes that control the rate of activity in cells and tissues).
A laboratory test can be done to measure the amount of T3 in your blood.
A blood sample is needed.
Your health care provider will tell you if you need to stop taking any medicines before the test that may affect your test result. DO NOT stop taking any medicine without first talking to your provider.
Drugs that can increase T3 measurements include:
Drugs that can decrease T3 measurements include:
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
This test is done to check your thyroid function. Thyroid function depends on the action of T3 and other hormones, including thyroid-stimulating hormone (TSH) and T4.
Sometimes it can be useful to measure both T3 and T4 when evaluating thyroid function.
The total T3 test measures the T3 that is both attached to proteins and floating free in the blood.
The free T3 test measures only the T3 that is floating free in the blood. The tests for free T3 are generally less accurate than for total T3.
Your provider may recommend this test if you have signs of a thyroid disorder, including:
The range for normal values are:
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different specimens. Talk to your provider about the meaning of your specific test results.
Normal values are age specific for people less than age 20. Check with your provider about your specific results.
A higher-than-normal level of T3 may be a sign of:
A high level of T3 may occur in pregnancy (especially with morning sickness at the end of the first trimester) or with the use of birth control pills or estrogen.
A lower-than-normal level may be due to:
Selenium deficiency causes a decrease in the conversion of T4 to T3, but it is not clear that this results in lower than normal T3 levels in people.
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person 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:
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Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.BACK TO TOP
Review Date: 1/9/2022
Reviewed By: Robert Hurd, MD, Professor of Endocrinology and Health Care Ethics, Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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