Frequently asked question: What is T4?

T4 is thyroxine. T4 is a hormone that is produced by the thyroid gland. Some of the hormone produced is stored in the thyroid gland, and the rest is found in the bloodstream. In the blood, most of the T4 is bound to proteins (such as TBG), yet a small percentage of it flows around as 'free T4.' T4 is actually not active; it becomes active only after it is converted into T3. Purpose: to test the function of the thyroid gland.

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