Is FT4 useful for Sport?

Based on the provided context, there is no direct evidence addressing the utility of FT4 (free thyroxine) for sport. FT4 is a standard thyroid function marker, but its role in sports performance optimization is limited. In asymptomatic athletes without clinical suspicion of thyroid dysfunction, a single FT4 measurement provides little actionable information for training or nutrition. Thyroid hormones influence basal metabolic rate and muscle metabolism, but routine FT4 testing for performance enhancement lacks strong evidence. Current guidelines recommend FT4 only when hypothyroidism or hyperthyroidism is suspected. Additionally, FT4 levels fluctuate diurnally and can be transiently altered by acute exercise. For athletes, more relevant biomarkers include iron status, vitamin D, or creatine kinase. Without specific symptoms, FT4 testing is not considered a useful sports marker. Evidence: practice/unclear. Caveat: No direct sources in context; based on general clinical practice.