Free leptin index and thyroid function in male highly trained athletes

Objective: Exercise training may cause changes in thyroid function. This thyroid response may be due to exercise-induced modulation of energy metabolism but also of the adipocytes endocrine function. In particular, the role of leptin and of circulating soluble leptin receptor (sOB-R) was unexplored. The aim of this study was to assess the relationships between thyroid function, whole body energy metabolism, and adipokines – mainly leptin and its receptor, sOB-R. Methods: We measured serum TSH, free tri-iodothyronine (FT3), free thyroxine, leptin, and sOB-R and assessed energy homeostasis by means of indirect calorimetry, in 27 highly trained athletes and 27 sedentary, healthy men. Results: TSH–FT3 ratio was lower in athletes (P<0.03), either in sustained power or anaerobic power-sprint athletes (n=13) or marathon runners (n=14). Whole body respiratory quotient was lower in athletes. Fasting serum sOB-R was higher and leptin lower in athletes than controls. Also serum adiponectin, resistin, and retinol binding protein-4 concentrations were different in athletes than in controls. The ratio between leptin and sOB-R, the free leptin index (FLI), was lower in athletes than in controls (0.025±0.014 vs 0.085±0.049; P<0.001). In multivariate analysis, FLI retained independent association with TSH–FT3 ratio. Conclusion: Male, elite athletes had lower TSH–FT3 ratio and FLI than controls while FLI was independently associated with TSH–FT3 ratio supporting the hypothesis that the level of biologically active leptin is involved in the adaptive response of thyroid function in professional athletes.