A free T4 (FT4) blood test is a laboratory test that measures the level of free thyroxine (T4) in the blood. T4 is a hormone produced by the thyroid gland that plays a key role in regulating metabolism. The FT4 test is used to assess thyroid function and to help diagnose and monitor conditions such as hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid). The FT4 test measures the amount of free T4 in the blood, which is the portion of T4 that is not bound to proteins and is available to be used by the body. The test is typically done by drawing a small amount of blood from a vein in the arm and sending it to a laboratory for analysis.
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