T4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of free T4 in your blood.
A blood sample is needed.
Your health care provider will tell you if you need to stop taking any medicines that may affect the test result. In general, test results are not affected by other medicines you may be taking.
Pregnancy and some diseases, including kidney and liver disease, can affect the results of this test.
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.
Your provider may recommend this test if you have signs of a thyroid disorder, including:
This test is also used to monitor people who are being treated for thyroid problems.
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 provider about the meaning of your specific test results.
To fully understand results of the free T4 test, results of other thyroid blood tests, such as TSH or T3, may be needed.
Test results may also be affected by pregnancy, estrogen level, liver problems, more severe body-wide illnesses, and inherited changes in a protein that binds T4.
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 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:
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.