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
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
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:
- Birth control pills
- Certain herbal remedies
Drugs that can decrease T3 measurements include:
- Anabolic steroids
- Antithyroid drugs (for example, propylthiouracil and methimazole)
How the Test will Feel
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.
Why the Test is Performed
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:
- Disorder in which the pituitary gland does not produce normal amounts of some or all of its hormone
- Overactive thyroid gland
- Underactive thyroid gland
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.
What Abnormal Results Mean
A higher-than-normal level of T3 may be a sign of:
- Overactive thyroid gland (for example, Graves disease)
- T3 thyrotoxicosis (rare)
- Toxic nodular goiter
- Taking thyroid medicines or certain supplements
- Liver disease
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:
- Short- or long-term illness
- Thyroiditis (swelling or inflammation of the thyroid gland -- Hashimoto disease is the most common type)
- Underactive thyroid gland
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)
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.
Reviewed By: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.