Free T4 thyroid hormone (FT4)
T4 is the inactive form of T4 that is bound to protein. Free T4 is the unbound T4 in the blood.

Thyroxine is the main thyroid hormone produced by the thyroid. Free thyroxine is converted to the more potent FT3 and is selenium and zinc dependent. FT4 is fairly inactive in the body.

Only 25% of the FT4 is in the blood, with 75% stored in brain and muscle. It is also the main prescription medication administered to hypothyroid patients in Australia (thyroixine/levothyroixine).

Serum (Blood Draw) or Blood Spot

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