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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.
Triiodothyronine; T3 radioimmunoassay
A blood sample is needed.
Your health care provider will tell you if you need to stop taking medicines that may affect the test result.
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. For example, in some cases of hyperthyroidism, T3 may be increased but T4 may be normal.
The T3 test measures the T3 that is both attached to proteins and floating free in the blood.
Your doctor may order this test if you have signs of a thyroid disorder, including:
The range for normal values is 100 - 200 nanograms per deciliter (ng/dL).
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different specimens. Talk to your doctor about the meaning of your specific test results.
A higher-than-normal level of T3 may be a sign of:
A high level of T3 may occur in pregnancy or with the use of birth control pills or estrogen.
A lower-than-normal level may be due to:
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:
Guber HA, Farag AF. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 24.
Salvatore D, Davies TF, Schlumberger MJ, et al. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, et al., eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 11.