How are FT3 and Sport related?

FT3 (free triiodothyronine) is the active thyroid hormone and a key regulator of energy metabolism. Exercise and physical activity can influence the thyroid axis: moderate endurance exercise may transiently increase FT3 levels, whereas intense overtraining is associated with decreased FT3 and a 'low-T3 syndrome.' Individual responses vary widely depending on training status, caloric balance, and genetic factors. Persistently low FT3 in athletes may indicate insufficient energy availability (RED-S). The evidence is largely based on observational studies and physiological mechanisms; there are no strong randomized trials proving a direct causal effect of sport on FT3. Single measurements are also subject to diurnal variation. Anyone interpreting FT3 in the context of sport should consider multiple measurements and the full clinical picture.