Objective: Women of advanced reproductive age are known to demonstrate subtle FSH elevations (monotropic FSH rise) while still retaining ovulatory function. The purpose of this study was to investigate the hypothesis that the physiologic basis for the monotropic FSH rise is an alteration in the secretion pattern of the GnRH pulse generator.
Methods: The subjects were 11 normal women age 40-45 years who underwent 24 hours of frequent blood sampling in the follicular (EF) and/or midluteal (ML) phases of spontaneous menstrual cycles. The controls were 11 normal women age 20-25 years. The respective gonadotropin secretion patterns were analyzed for LH pulse frequency, mean LH and FSH levels, and LH pulse amplitude.
Results: There were no differences between the groups for estradiol (E2) and progesterone when the respective cycle phases were compared. The 24-hour mean FSH level was significantly increased in the older women in both the EF and ML phases. There were no differences between the groups in either cycle phase for LH pulse frequency, LH pulse amplitude, and mean LH levels.
Conclusion: The results lend no support to the hypothesis that a slowing or other alteration of the GnRH pulse generator is the basis for the monotropic FSH rise in older ovulatory women. Other possibilities include the dynamics of E2 secretion or changes in FSH-modulating peptides (i.e., inhibin) in these women.