Decremental follicle-stimulating hormone and dominant follicle development during the normal menstrual cycle.

Objective: To study the relationship between decreasing serum FSH levels in the follicular phase of the normal menstrual cycle and follicle development by means of transvaginal sonography and E2 production.

Design: Daily blood samples were taken and transvaginal sonography was performed every other day in 16 normal regularly cycling female volunteers.

Main Outcome Measure: Serum levels of FSH, LH, E2, and P and follicle diameter by transvaginal sonography.

Results: A distinct variability in individual maximal early follicular phase FSH plasma levels was observed (range 4.4 to 11.2 mIU/mL [conversion factor to SI unit, 1.000]). Differences in maximum FSH levels did not correlate with other endocrine or sonographic follicular phase characteristics. The follicular phase FSH decrease (from median 6.6 to 2.9 mIU/mL [conversion factor to SI unit, 1.000]) took place between cycle day 5 and 13 and was linear (0.5 +/- 0.05 mIU/mL per day; mean +/- SD). A significant correlation was found between serum FSH decrease and E2 increase. The day of sonographic appearance of a dominant follicle (median cycle day 8) did correlate with the first rise of the E2 plasma concentration.

Conclusions: The present study suggests that even in women exhibiting normal ovarian function a 2.5-fold difference in FSH threshold concentrations for follicle recruitment does occur. Moreover, the magnitude of decrease in serum FSH concentrations during the follicular phase affects dominant follicle E2 production. The sonographic appearance of the dominant follicle is associated with a rise in serum E2 levels.