THYROXINE, FREE (Free T4)

Total T4 and free T4 (FT4) are two separate tests that can help the doctor evaluate thyroid function. The total T4 test was used for many years to help diagnose hyper- and hypothyroidism. It is a useful test but can be affected by the amount of protein available in the blood to bind to the hormone. The free T4 test is a newer test that is not affected by protein levels. Since free T4 is the active form of thyroxine, it is thought to be a more accurate reflection of thyroid hormone function and, in most cases, its use has replaced that of the total T4 (Hagerstown Medical Laboratory no longer offers total T4). Thyroxine measurment is usually ordered along with or following a TSH test. This helps the doctor to determine whether the thyroid hormone feedback system is functioning as it should, and the results of the tests help to distinguish between different causes of hyper- and hypothyroidism. Sometimes a T3 test will also be ordered to give the doctor additional diagnostic information.

A FT4 and TSH test may be ordered to help evaluate a patient with a goiter and to aid in the diagnosis of female infertility. If the doctor suspects that the patient may have an autoimmune-related thyroid condition, thyroid antibodies may be ordered along with a FT4. In patients with known thyroid dysfunction, FT4 and/or TSH tests may be ordered to monitor thyroid function.