Immunoreactive inhibin levels during ovarian stimulation may predict granulosa cell maturity

The aim of the study was to assess whether the immunoreactive inhibin response to ovarian stimulation in polycystic ovarian syndrome was of predictive value for the outcome of ovulation induction.

Daily injections of purified FSH (Metrodin, Serono Laboratories, UK) were administered for the purpose of inducing development and ovulation of a single follicle.

All patients had anovulatory infertility secondary to polycystic ovarian syndrome and were resistant to clomiphene citrate.

Alternate day serum samples were obtained for measurement of gonadotrophins, sex steroids and inhibin by radioimmunoassay. Alternate day ovarian ultrasound scans were carried out to monitor follicular development.

There was a high incidence of multiple follicular development (MFD) (10.2 +/- 7.1 (standard deviation) follicles). There was a close relationship between the number of follicles on the day of the ovulatory trigger and serum oestradiol (E2) levels (R = 0.97, P < 0.001). This relationship was not seen with serum immunoreactive inhibin levels (R = 0.31). The cycles were divided into two groups depending whether there was or was not MFD (more than five follicles > 7 mm in diameter on day of hCG administration). The MFD group had 15.6 +/- 8.5 follicles and the non-MFD group 3.2 +/- 1.6 follicles. There was no difference in the circulating E2 concentration (pmol/l) per follicle between the two groups (non-MFD cycles, n = 6,345 +/- 28.9; MFD cycles, n = 8, 308 +/- 40.29). However, the immunoreactive inhibin concentration (U/ml) per follicle was lower, P < 0.001, in the MFD group (4.3 +/- 1.6 vs 6.9 +/- 0.5). The maximum and mean follicle diameter were lower, P < 0.01, in the MFD group (maximum follicle size 16.5 +/- 3.9 vs 21.2 +/- 1.5 mm, mean follicle size 11.5 +/- 2 vs 14.9 +/- 2.3). When individual cycles were examined E2 and immunoreactive inhibin secretion rose in parallel in cycles with less than five follicles with a rapid rise occurring when the follicles reached about 12 mm in diameter. In cycles where there was MFD there was a disparity between E2 and immunoreactive inhibin secretion with E2 levels rising 4.3 +/- 1.4 days before immunoreactive inhibin levels.

These data suggest that in cycles where there are multiple small follicles, E2 secretion is maintained whereas immunoreactive inhibin secretion is substantially lower. Thus, in view of the disparity between E2 and immunoreactive inhibin secretion evident in the MFD group, measurement of immunoreactive serum inhibin concentration may be a better indicator of granulosa cell maturity. Immunoreactive inhibin secretion may occur only from healthy mature follicles.