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.
A blood sample is needed. For information on how this is usually done, see: Venipuncture
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:
Drugs that can decrease T4 measurements include:
This list may not include all medications.
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.
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:
A typical normal range is 4.5 to 11.2 micrograms per deciliter (mcg/dL).
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
Greater than normal levels of T4 may be due to conditions that involve an overactive thyroid, including:
Lower than normal levels 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:
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.