The results are consistent with a role for serum immunoreactive inhibin, in addition to oestradiol, in the regulation of FSH during the follicular phase of the menstrual cycle as a function of increasing age. This is postulated to reflect diminished folliculogenesis as age progresses with the known decline in the numbers of primordial follicles in the ovary as the menopause approaches.

No abstract

Inhibin levels fall rapidly post partum and remain low until close to the time of resumption of follicular activity and menses. The post partum rise in serum FSH appears to be much more closely related to falling oestradiol levels than to the very early and rapid fall in inhibin. Oestradiol thus appears to be the predominant negative feedback factor influencing FSH secretion during the post partum period. The low inhibin levels may allow FSH to rise to levels high in the follicular phase range under the predominant negative feedback control of oestradiol. Inhibin levels do not appear to be a suitable marker of returning fertility.

To determine whether FSH is a physiological regulator of the serum immunoreactive inhibin (INH) concentration during the follicular phase of the normal menstrual cycle, purified FSH (Metrodin) was administered in doses of 100 IU (n = 6), 150 IU (n = 5), and 200 IU (n = 5) to normal, regularly cycling volunteers between days 3-7 of the menstrual cycle. A control group (n = 5) received normal saline. There was a linear dose-related increase in serum INH (and in serum FSH) in response to the three doses of FSH, with 200 IU leading to a 107% increase in INH and a 68% increase in FSH. Serum estradiol rose in response to the two higher doses of FSH. There was a significant correlation between the actual increases in INH and estradiol (r = 0.53; P < 0.01). It was concluded that FSH stimulates INH in the follicular phase of the normal menstrual cycle, consistent with a physiological role for FSH in the regulation of granulosa cell production of inhibin.

It was concluded that both FSH and LH are capable of modulating inhibin production during the luteal phase of the menstrual cycle. FSH may exert its actions on the corpus luteum or alternatively on developing follicles. The present study cannot clearly distinguish between these possibilities.