Sexual dimorphism in circulating monomeric and dimeric inhibins in normal boys and girls from birth to puberty

BACKGROUND AND OBJECTIVE Inhibins are peptides, mainly of gonadal origin, t hat suppress FSH production. Dimeric forms of inhibin (A and B) have been p roposed as peripheral markers of Sertoli and granulosa cell function, The a im of this study was to establish the relationship between circulating dime ric and monomeric inhibins, and gonadotrophins and sex steroids, in normal boys and girls from birth to puberty. SUBJECTS One hundred and forty-six normal children (females: 57; males: 89) were studied. MEASUREMENTS Serum LH and FSH were measured by an immunofluorometric assay. Serum oestradiol and testosterone were measured by radioimmunoassay. Serum inhibin A and B, and Pro-alpha C, were measured by specific two-site enzym e-linked immunosorbent assays. RESULTS In boys from birth to 6 months of ag e, the mean serum inhibin a concentration was as high (477 +/- 53.7 ng/l) a s that found at puberty (400 +/- 70.2 ng/l). After the first year, inhibin B gradually decreased to reach its lowest concentration (153 +/- 23.6ng/l) at age 4-6 years. At approximately age 10, it rose progressively to reach p ubertal concentrations, Pro-alpha C showed a similar pattern but at lower c oncentrations. Inhibin A was not detected at any age, In girls from birth t o 6 months, inhibin B levels (83.0 +/- 18.3 ng/l) were approximately 50% lo wer than those found at puberty (181 +/- 25.7 ng/l). After 6 months of age, these levels dropped (17.5 +/- 1.6 ng/ I) and remained low until the prepu bertal years, Thereafter, they increased to pubertal concentrations, The se rum inhibin A concentration after birth (29.9 +/- 8.7 ng/l) was similar to that found at puberty (18.3 +/- 5.7 ng/l); after 6 months, it fell to undet ectable levels until the prepubertal years. CONCLUSION The sex difference in serum levels of gonadotrophins is associat ed with sex differences in the levels and proportions of circulating dimeri c and monomeric inhibins.