Thyroid hormones as permissive regulators of skeletal muscle protein accretion and oxidative capacity: Implications for resistance and endurance training.

Thyroid hormones (THs) are fundamental regulators of skeletal muscle energy metabolism and protein turnover. Instead of directly stimulating muscle growth, they create the intracellular environment required for effective adaptation to exercise. Adequate TH availability supports mitochondrial integrity, ribosome biogenesis, and coordinated protein turnover, thereby enabling the translation of anabolic signaling into structural adaptation and remodeling. Accordingly, reduced translational efficiency during hypothyroidism may blunt hypertrophic responses. In contrast, hyperthyroidism accelerates proteolytic pathways and compromises net protein gain despite elevated turnover. Training modalities determine the physiological outcome within this endocrine context, with endurance exercise primarily enhancing oxidative remodeling, while resistance training, when sufficient biosynthetic capacity is present, can stimulate myofibrillar accretion. Reduced energy availability lowers T3 concentrations and may attenuate adaptive efficiency across both modalities. Overall, thyroid status emerges as a critical determinant of muscle remodeling capacity, underscoring the clinical relevance of thyroid assessment in exercise-based interventions.