What role does FT3 play in Sport?

FT3 (free triiodothyronine) is the active thyroid hormone and a key regulator of metabolism, protein synthesis, and mitochondrial function—all critical for athletic performance. Optimal FT3 levels support endurance, muscle strength, and recovery. Overtraining or chronic calorie deficit can suppress FT3, leading to fatigue and reduced performance. However, FT3 is not a routine sports biomarker in consumer tests; its levels fluctuate diurnally and are influenced by diet, sleep, and training load. MyBody-X does not measure FT3. The evidence linking FT3 to sport performance is mechanistic and observational, not from controlled trials. Athletes should not rely on single FT3 readings for training decisions. Clinical assessment is needed if thyroid dysfunction is suspected.