Evidence check: FT4 in the context of Sport

Thyroid function, as measured by free thyroxine (FT4), plays a role in energy metabolism and can influence athletic performance. However, there is no strong evidence from randomized controlled trials supporting FT4 as a standalone biomarker for sports optimization. Most data come from observational studies showing associations between thyroid hormones and training adaptations, but not causal effects. Moreover, FT4 reference ranges and individual values vary widely. A single FT4 measurement without TSH and fT3 is of limited value. For athletes, maintaining normal thyroid status is important, but deliberately manipulating FT4 through supplements or medication is not evidence-based and may be risky (e.g., thyrotoxicosis). The evidence level is therefore 'mechanistic/observational'.