Early activation of the inhibin B/FSH axis in obese Tanner stage G1PH1 boys

Objective: To determine whether early activation of the inhibin B/FSH axis is detectable in prepubertal obese boys. Methods: Thirty-five simple obese Tanner stage G1PH1 boys with body mass index over 25 aged 8-11 years old and 25 age-matched nonobese healthy prepubertal boys (G1PH1) were clinically examined and testicular size measured by ultrasound. Serum inhibin B, testosterone, LH, FSH, dehydroepiandrosterone (DHEA), DHEA sulfate (DHEAS) and bone age were measured. GnRH-stimulating tests were performed in the obese children and the relationships between inhibin B and bone age, testicular volume, DHEA, DHEAS, and stimulated peak LH, FSH and testosterone were analysed. Results: The majority of basal LH and testosterone levels were undetectable in both groups of G1PH1 children and no difference was apparent between the groups. However, testicular volume (left 1.21 ml vs 0.83 ml, right 1.15 ml vs 0.81 ml), bone age, DHEA and DHEAS levels were significantly higher in obese children. Inhibin B was detectable in all children. Basal levels were significantly higher in obese children (103.3 ng/l vs 60.95 ng/l, P < 0.001) and correlated with testicular volume (left: r_s = 0.655, right: r_s = 0.638, P < 0.001) and bone age (r_s = 0.554, P < 0.05). Basal FSH levels did not correlate with inhibin B. However, after GnRH stimulation, a clear negative correlation between peak FSH and basal inhibin B was apparent (r_s = -0.583, P < 0.001) consistent with early activation of the inhibin B/FSH axis. Conclusions: Activation of the inhibin B/FSH axis is apparent in obese Tanner stage G1PH1 boys and appears to represent an early hormonal change of puberty in these individuals.