What role does FT4 play in Sport?

FT4 (free thyroxine) is the biologically active thyroid hormone that regulates basal metabolic rate, protein synthesis, and mitochondrial function. In sports, FT4 indirectly influences endurance performance, muscle recovery, and fat metabolism. Optimal FT4 levels are essential for normal cardiovascular function and energy provision. However, there is no robust evidence establishing FT4 as a direct performance marker or target for athletic optimization. Thyroid dysfunction (hyper- or hypothyroidism) can impair training capacity, but in healthy athletes FT4 typically remains within reference ranges. The evidence is mechanistic: the physiological role is well understood, but the direct utility for sport-specific guidance or personalization is weak. An FT4 test alone is insufficient; TSH and fT3 should be measured concurrently. Caveat: No specific sport studies were provided in the context.