Homologous in vitro bioassay for follicle-stimulating hormone (FSH) reveals increased FSH biological signal during the mid- to late luteal phase of the human menstrual cycle.

Monitoring of the secretory dynamics of FSH during the human menstrual cycle has demonstrated conflicting results between the amounts of FSH measured by dimer-specific immunoassays and previous heterologous in vitro bioassays. These differences suggest somewhat different models of the steroidal and nonsteroidal regulation of FSH secretion and its control of folliculogenesis in the human. We have developed a homologous in vitro bioassay, using the recombinant human FSH receptor cotransfected into Chinese hamster ovary cells with a cAMP-responsive luciferase reporter gene, that overcomes many of the theoretic shortfalls of previous assays and allows reevaluation of the changes in bioactive FSH across the menstrual cycle. Bioactive FSH levels measured across 12 menstrual cycles in 11 normal women ranged from 4-40 IU/L. FSH bioactivity was constant during the menstrual cycle, with elevations noted only during the mid- to late luteal phase. Bioactive FSH levels were similar to immunoactive FSH levels across the cycle as indicated by a ratio of bioactive to immunoreactive FSH (FSH B/I) of 1.10 +/- 0.04 across the follicular and early luteal phases. However, during the mid- to late luteal phase, the mean FSH B/I rose to 1.65 +/- 0.07, which significantly exceeded that during the rest of the cycle (P < 0.001). This change in FSH B/I occurs at a critical time during folliculogenesis when the next cohort of follicles is being recruited and appears to be secondary to a decrease in immunoreactive FSH unaccompanied by a similar decrease in in vitro bioactivity. There was good agreement between immunoassay and bioassay results on the day of the midcycle gonadotropin surge (FSH B/I = 1.07 +/- 0.14), which was not different from that in the follicular phase (days -17 to -2; FSH B/I = 1.06 +/- 0.05) or the FSH B/I measured in postmenopausal women (0.93 +/- 0.2). These observations using a novel homologous human FSH in vitro bioassay indicate that bioactive FSH levels are not declining during the time of active corpus luteum formation and secretory activity. Thus, there is a previously undetected increased biologic signal during the mid- to late luteal phase, suggesting that the influence of elevated FSH on the cohort of developing follicles (including the subsequent dominant follicle) begins earlier during the luteal-follicular transition than previously predicted by FSH immunoreactivity.