T4 (thyroxine) is a hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of T4 in your blood.
How the test is performed
A blood sample is needed. For information on how this is usually done, see: Venipuncture
How to prepare for the test
Your health care provider will tell you, if necessary, to stop taking drugs that may interfere with the test.
Drugs that can increase T4 measurements include:
Birth control pills
Drugs that can decrease T4 measurements include:
Antithyroid drugs (for example, propylthiouracil and methimazole)
This list may not include all medications.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. You may feel some throbbing after the test.
Why the test is performed
This test is done to check your thyroid function. Thyroid function is complex and depends on the action of many different thyroid hormones, including thyroid-stimulating hormone (TSH) and T3 (triiodothyronine).
Your doctor may order this test if you have signs of a thyroid disorder, including:
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:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 233.
Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, Larsen PR, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier;2011:chap 11.
Shehzad Topiwala, MD, Chief Consultant Endocrinologist, Premier Medical Associates, The Villages, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine, and David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.