Functional data analysis of FSH and inhibin B levels in four ovulatory cycles shows a changing oscillatory interaction during the follicular phase.

Objective: The pituitary hormone FSH (follicle-stimulating hormone) and the follicular granulosa cell hormone inhibin B are important in follicle development ending in ovulation. We hypothesized that FSH and inhibin B interact by oscillatory feedback during at least part of the follicular phase. Methods: Daily serum FSH and inhibin B were measured throughout the ovulatory cycles of four women. Transvaginal sonography was performed to establish the last day of the dominant follicle (DF) and the following day of its collapse. Serum FSH and inhibin B levels were indexed to day 0, the day of the ovulatory inhibin B peak, which occurred on the day of DF collapse in three cycles and the last day of the DF in one cycle. The out-of-phase day-specific serum FSH and inhibin B levels from the cycles were aligned with functional data analysis (fda) to generate standardized mean day-specific levels for meaningful comparison of FSH and inhibin B as a function of time-of-cycle. Results: The curves for the standardized mean day-specific FSH and inhibin B levels were related. A maximum positive correlation between FSH and inhibin B was achieved when the FSH curve was translated forward by 1 day relative to the inhibin B curve (curves were then "in-phase"). Similarly, translating the inhibin B curve forward by 1 day relative to the FSH curve resulted in a maximum negative correlation (curves were then "out-of-phase"). An oscillatory positive-negative feedback was discernible between FSH and inhibin B during the early follicular phase which appeared to change after day -8, the approximate timing for DF selection. Conclusions: Examining the relationship between inhibin B and FSH by fda using individual cycles has the potential to remove the obscuration that occurs from averaging multiple cycles. This exploratory study with fda revealed a possible FSH and inhibin B oscillatory feedback,