T4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of T4 in your blood.
A blood sample is needed. For information on how this is usually done, see: Venipuncture
Your health care provider will tell you if you need to stop taking medicines that may affect the test result.
Drugs that can increase T4 measurements include:
Drugs that can decrease T4 measurements include:
This list may not include all medications that affect T4.
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 different thyroid hormones, including T4, thyroid-stimulating hormone (TSH), and T3 (triiodothyronine).
Your doctor may order this test if you have signs of a thyroid disorder, including:
A typical normal range is 4.5 to 11.2 micrograms per deciliter (mcg/dL).
Normal value ranges may vary slightly among different laboratories. Some laboratories use different measurements or may test different specimens. Talk to your doctor about the meaning of your specific test results.
A higher than normal level of T4 may be due to conditions that involve an overactive thyroid, including:
A lower than normal level of T4 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:
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.