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Summary

Objective  Sex steroid concentrations in men are related to body composition and both are determined by genetic and environmental factors. This study investigates heritability estimates of sex steroid serum concentrations and body composition as well as the genetic and environmental components of their interrelation.

Patients  Six hundred and seventy-four men (25–45 years) were included in this study with 274 independent pairs of brothers.

Measurements  Body composition and regional fat mass estimates were determined using dual-energy X-ray absorptiometry. Serum testosterone (T), SHBG, oestradiol (E2) and LH levels were determined by immunoassay; free T and E2 levels were calculated.

Results  Both sex steroid hormone concentrations and indices of body composition exhibited significant heritability estimates. Among sex steroid hormones, T had the highest heritability (h2 = 0·65), followed by free T (h2 = 0·54). A heritability of 0·73 was observed for SHBG; a heritability estimate of 0·83 was obtained for body weight. Significant genetic correlations were found between whole body fat mass and serum T (ρG = –0·46), free T (ρG = –0·27) and SHBG (ρG = –0·48) concentrations. No genetic relationship was observed between total (F) E2 or LH concentrations, respectively, and body composition.

Conclusion  Both sex steroid serum levels and body composition are under strong genetic control. Their interrelation is in part underlied by a genetic correlation, indicative of the action of shared genes.