What to watch with FT3 and Sport?

FT3 (free triiodothyronine) is the active thyroid hormone and directly influences metabolism, making it relevant for athletes. Overtraining or chronic low energy availability can suppress FT3 levels (non-thyroidal illness syndrome), leading to fatigue, poor recovery, and reduced performance. Conversely, elevated FT3 (hyperthyroidism) may cause tachycardia, heat intolerance, and muscle weakness. Important: FT3 must be interpreted together with TSH and FT4; isolated FT3 changes are rarely actionable. For athletes, monitoring thyroid function is advisable if symptoms like persistent fatigue, weight changes, or palpitations occur. Evidence for direct FT3 optimization via diet or supplements is limited; focus on adequate caloric and carbohydrate intake, stress reduction, and sleep. Caveat: Do not self-prescribe thyroid hormones. Always consult an endocrinologist for abnormal values.