Body composition and metabolic parameters are associated with variation in thyroid hormone levels among euthyroid young men

Objective: Thyroid disorders affect metabolism and body composition. Existing literature has been conflicting on whether this is also the case for thyroid hormone levels within the euthyroid range. Therefore, we have investigated the relationship between thyroid hormone concentrations and body composition together with metabolic parameters in a population of healthy euthyroid men. Methods: Healthy male siblings (n = 941, 25-45 years, median BMI 24.6) were recruited in a cross-sectional, population-based study; a history or treatment of thyroid disease and thyroid autoimmunity were exclusion criteria. Body composition and muscle cross-sectional area were assessed by dual-energy X-ray absorptiometry and peripheral quantitative computed tomography. Total (triiodothyronine (T-3; TT3) thyroxine and (T-4; TT4)) and free thyroid hormones (FT3 and FT4), TSH, and reverse T-3 (rT3) and thyroid-binding globulin (TBG) were determined using immunoassays. Results: BMI was positively associated with (F)T-3 (P < 0.0001). Whole body fat mass displayed positive associations with TT3 and with (F)T-4 and TBG (P <= 0.0006). Positive associations were further observed between leptin and (F)T3, TT4, and TBG (P <= 0.0002). Inverse associations between lean mass and muscle cross-sectional area and (F)T-3, (F)T-4, and TBG were observed (P <= 0.0003). Higher levels of (F)T-3 and TBG were associated with lower insulin sensitivity, assessed by homeostatic model assessment of insulin resistance (IR; P <= 0.0001). No associations between TSH and body composition or metabolic parameters were seen. Conclusion: We show that a less favorable body composition (with higher fat and lower muscle mass and accompanying higher leptin concentrations) and IR are associated with higher thyroid hormone levels in healthy young men with well characterized euthyroidism.