Free T4 (thyroxine) is the primary hormone produced by the thyroid gland. It serves as the precursor to the more active T3 hormone. Free T4 levels help distinguish between different types of thyroid dysfunction and are important for patients already on thyroid medication.
In the context of TRT, free T4 is primarily useful as part of a complete thyroid panel to ensure the thyroid isn't contributing to fatigue and metabolic symptoms.