The role of inhibin in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a common heterogeneous disorder which, in its severest manifestations, is associated with anovulation, hyperandrogenism and metabolic imbalance. The biochemical markers for the condition can include a significantly raised LH:FSH ratio and a raised testosterone concentration, indicating a derangement of the hypothalamo--pituitary--ovarian axis which may be primary or secondary to a primary ovarian pathology. The bioactive inhibins are heterodimeric glycoproteins consisting of alpha-betaA (inhibin A) and alpha-betaB (inhibin B) subunits. They play an endocrine role in co-regulating (with oestradiol) the suppression of FSH during the late follicular and luteal phases of the ovarian cycle and they are implicated in intraovarian paracrine signalling. Inhibin B, which is the predominant form in small pre-ovulatory follicles, increases in concentration from early in the follicular phase to reach a peak coincident with the onset of the decrease in FSH which forms the basis of the pattern of mono-ovulation seen in normo-ovulatory women. Several unique features of the dysovulation of women with PCOS, namely their failure to recruit and develop a dominant follicle despite having 'normal' concentrations of endogenous FHS, the raised LH:FSH ratio and their exquisite sensitivity to exogenous FSH injections, may be explained by their significantly higher inhibin B concentrations. Studies into inhibin B parameters in women with PCOS demonstrate that women with anovular PCOS have significantly higher concentrations of circulating inhibin B and that they lack the pulsatile pattern of secretion that can be detected in normo-ovulatory women during the mid-follicular phase. The inhibin B response to ovulation induction with clomiphene citrate in women with PCOS differs from that in normo-ovulatory women taking the antioestrogen. Women with PCOS who over-respond to ovulation induction with injected FSH in a 'low-dose' step-up protocol' and recruit multiple follicles have significantly higher concentrations of pre-treatment inhibin B than PCOS subjects who do not.