Evidence check: FT3 in the context of Sport

FT3 (free triiodothyronine) is the active thyroid hormone and plays a key role in energy metabolism, protein synthesis, and thermogenesis – all factors relevant to athletic performance. In sports, FT3 is sometimes measured to detect overtraining or a low-T3 syndrome caused by caloric restriction. However, the evidence for using FT3 as a direct performance marker is moderate: observational studies show associations between low FT3 and reduced performance, but no RCTs prove causality. Individual variability is high, and FT3 is influenced by diet, stress, and sleep. Importantly, using thyroid hormones for performance enhancement is dangerous and can cause severe side effects. FT3 testing may be useful as part of a comprehensive sports medicine workup, but not as a standalone marker. Overall evidence level: moderate to weak.