How are FT4 and Sport related?

The relationship between FT4 (free thyroxine) and sport is complex and not fully understood. FT4 is a thyroid hormone that regulates basal metabolic rate and energy expenditure. Moderate, regular exercise tends to support stable thyroid function, while high-intensity endurance training or overtraining can lead to decreased FT4 and fT3 levels – a phenomenon known as 'low T3 syndrome' or 'euthyroid sick syndrome'. This is considered an adaptive response to energy deficit and physiological stress, not a clinical thyroid disorder. Acute exercise may transiently lower FT4, but chronic effects depend on training load, nutrition, and recovery. Importantly, an isolated low FT4 without elevated TSH does not indicate hypothyroidism requiring treatment. In athletes presenting with fatigue or performance decline, thyroid markers should be evaluated together with TSH, fT3, and cortisol. Evidence comes mainly from observational studies and physiological reasoning; robust RCTs are lacking. MyBody-X measures FT4 in blood, but interpretation requires clinical context. Caveat: supplements or diets claiming to 'optimize' thyroid function are generally not evidence-based.