Endocrine Abstracts

Introduction: Thyroid disorders affect body composition; however, few studies have addressed the effect of variation of thyroid hormone status within the euthyroid range on body composition. Therefore, we investigated the relationship between indices of thyroid status and body composition in a population of euthyroid men.

Methods: Healthy male siblings (n=677, 25–45 years, mean BMI 25.2) were recruited in a cross-sectional, population-based study; a history or treatment of thyroid disease and positive thyroid auto-immunity were exclusions. Body composition was determined using DXA, total (TT₃, TT₄) and free thyroid hormones (FT₃, FT₄), TSH, TBG and rT₃ using immunoassays.

Results: Both FT₃ and TT₃ were positively associated with weight, BMI and fat mass (all P values between 0.0008 and 0.02), whereas FT₄ and TT₄ were positively associated with fat mass only (P=0.02). Inverse associations of lean mass with TT₃ (P=0.01) and FT₄ were observed (P=0.04). We observed no association of TSH with body composition. Concordant positive associations were observed for TT₃, rT₃ (P=0.006) and TBG (P<0.001) with weight and fat mass, and inverse associations with lean mass (both P<0.0001).

Conclusion: In our study, both free and total thyroid hormone concentrations and TBG were positively related to body weight and BMI (for T₃) and to fat mass (for T₃, T₄ and rT₃), and negatively to lean mass (TT₃, FT₄, rT₃ and TBG). The cross-sectional design does not allow establishing the direction of these associations. In contrast with prior reports, TSH was not associated with body composition in this population of non-obese young men with well characterized euthyroidism.