Thyroid Hormone Tests

T3 (Triiodothyronine) Test, T4 (Thyroxine) Test, Thyroxine (T4) Test, Triiodothyronine (T3) Test

Why It Is Done

Thyroid hormone tests are done to:

• Find out what is causing an abnormal thyroid-stimulating hormone (TSH) test. For more information, see the medical test Thyroid-Stimulating Hormone (TSH). This is the most common reason for thyroid hormone tests.
• Check how well treatment for thyroid disease is working. The total thyroxine (T4), free thyroxine (FT4), and free thyroxine index (FTI) values are often used to keep track of treatment for hyperthyroidism.
• Screen newborns to find out if the thyroid gland function is normal. A condition called congenital hypothyroidism can prevent normal growth and development and cause other severe problems, such as mental retardation, if it is not treated soon after birth.

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Author:	Caroline Rea, RN, BS, MS	Last Updated: December 3, 2007
Medical Review:	Caroline S. Rhoads, MD - Internal Medicine David C.W. Lau, MD, PhD, FRCPC - Endocrinology & Metabolism
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