CONTEXT In vitro and animal studies have reported conflicting results regarding an independent role for FSH in the regulation of bone turnover. OBJECTIVE Our objective was to test the hypothesis that suppressing serum FSH while holding serum gonadal steroid levels stable in the eugonadal range will affect biochemical markers of bone metabolism in healthy men. PARTICIPANTS, DESIGN, AND SETTING Eugonadal men aged 20 to 50 years participated in this randomized controlled trial at a tertiary care academic teaching hospital. INTERVENTIONS Participants received monthly GnRH analog injections to suppress FSH secretion plus daily topical testosterone gel in prespecified doses (intervention group). Controls received matching placebos (control group). Subjects in the intervention group were individually matched with subjects in the control group to ensure that the mean testosterone and estradiol levels (measured every 4 weeks during the 16-week study period) in the 2 groups were similar. MAIN OUTCOME MEASURES Biochemical markers of bone resorption (serum N-terminal telopeptide and C-terminal telopeptide), bone formation (serum osteocalcin), and FSH were measured at baseline and after 16 weeks of treatment. RESULTS Serum FSH declined by 2% in the control group and by 60% in the intervention group (P < .0001 for the between-group difference). Despite the substantial suppression of serum FSH in the intervention group, serum N-terminal telopeptide, C-terminal telopeptide, and osteocalcin did not change in the intervention group, nor were any between-group differences observed. CONCLUSION When gonadal steroid levels are held constant, short-to midterm suppression of FSH does not affect bone turnover in men. FSH does not appear to be a significant regulator of bone metabolism in eugonadal men.
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