EVIDENCE FOR A NEGATIVE FEEDBACK ROLE OF INHIBIN IN FOLLICLE-STIMULATING-HORMONE REGULATION IN WOMEN

The pituitary - ovarian axis describes a regulatory system in which pi tuitary gonadotrophins stimulate ovarian folliculogenesis and the prod uction of both steroid and protein hormones, which in turn exercise fe edback control on the pituitary and hypothalamus. The availability of a radioimmunoassay for serum inhibin has allowed exploration of inhibi n's role in the regulation of follicle stimulating hormone (FSH). This gonadotrophin stimulates ovarian inhibin secretion within the physiol ogical dose range, both after exogenous administration and during endo genous rises, as are seen during puberty and the mid-cycle FSH surge. No direct evidence of FSH suppression by inhibin administered to human subjects has been obtained. During periods of endogenously raised inh ibin levels, however, FSH is suppressed, as seen during the luteal pha se of the menstrual cycle (lutenizing hormone drives inhibin productio n at that time), during pregnancy and in patients with inhibin-secreti ng granulosa cell tumours of the ovary. In contrast, decreased inhibin levels in the presence of intact hypothalamo-pituitary function resul t in increased FSH concentrations-as in early childhood, during the lu teal - follicular transition and during reproductive ageing. After the menopause inhibin levels are normally undetectable, with FSH being ma rkedly elevated. These observations are compatible with a role for end ogenous inhibin in the regulation of FSH, though the contribution of o estradiol is in most instances impossible to dissect out separately.