Thyroid hormone concentrations and skeletal muscle metabolism during exercise in anorexic females.

Food restriction has been shown to result in an impairment of thyroid status, as the body attempts to conserve energy. In turn, hypothyroidism has been associated with impaired oxidative muscle metabolism, which could negatively affect one's ability to perform exercise. The purpose of this study was to examine the relationship between thyroid status and muscle metabolism during exercise in anorexic (n = 8) and control (n = 13) female subjects. Thyroid status was evaluated by measuring resting serum triiodothyronine (T3) and thyroxine (T4) concentrations, and muscle metabolism was evaluated by 31phosphorus magnetic resonance spectroscopy during plantar flexion exercise. Concentration of T3 and T4 in anorexics were significantly lower than in controls. The ratio of inorganic phosphate to phosphocreatine (Pi/PCr) was significantly higher in the anorexic compared with the control subjects at lower (p < 0.05), but not higher, relative exercise intensities. Rates of PCr recovery were not different between anorexics (tau = 50.2 +/- 15.8 s) and controls (tau = 42.6 +/- 16.7 s). In conclusion, despite lower circulating thyroid hormones in anorexic compared with control subjects, differences in oxidative metabolism between these groups were small.