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.
Thyroxine test; Total T4 test
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 you need to stop taking medicines that may affect the test result.
Drugs that can increase T4 measurements include:
- Birth control pills
Drugs that can decrease T4 measurements include:
- Anabolic steroids
- Antithyroid drugs (for example, propylthiouracil and methimazole)
- Glucocorticoid medications
- Interferon alpha
This list may not include all medications that affect T4.
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 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:
- Hyperthyroidism (overactive thyroid, in which the thyroid produces too much hormone)
- Hypothyroidism (underactive thyroid, in which the thyroid does not produce enough hormone)
- Hypopituitarism (the pituitary gland does not produce enough of its hormones)
- Thyrotoxic periodic paralysis (muscle weakness or paralysis caused by a high level of thyroid hormone)
- Thyroid nodule (lump in the thyroid)
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.
What Abnormal Results Mean
A higher than normal level of T4 may be due to conditions that involve an overactive thyroid, including:
- Graves disease
- Germ cell tumors
- High levels of the protein that carries T4 in the blood (can occur with pregnancy, use of birth control pills or estrogen, liver disease, and as part of an inherited condition)
- Iodine-induced hyperthyroidism
- Subacute thyroiditis
- Toxic multinodular goiter
- Trophoblastic disease
- Over-treatment with thyroid hormone medicine
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:
- 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, Kronenberg HM, 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.