Gonadal steroids and proteins exert direct pituitary effects on gonadotropin secretion along with their effects on GnRH secretion. Estradiol, for example, exerts both negative and positive feedback effects directly on the pituitary. Testosterone administration inhibits LH and FSH secretion by a direct pituitary effect in men.42 This effect is mediated in part by aromatization of testosterone to estradiol in vivo. Progesterone and testosterone also exert direct pituitary effects when studied in vitro, although these steroids have not been studied as definitively in vivo as has estradiol.

The major protein hormone product of the gonads is inhibin, a substance that has been studied for over 50 years,43 and now has been characterized structurally.44,45 Two 32-kDa forms of the glycoprotein, termed inhibin A and inhibin B and containing α and β subunits, have been described. Inhibin inhibits pituitary FSH secretion selectively, although at high levels it may also inhibit LH. Inhibin is produced in the Sertoli cells of the testis and in granulosa cells of the ovary, mainly under the stimulatory influence of FSH46,54