Age-Related Analysis of Inhibin A, Inhibin B

Relative to the Intercycle Monotropic Follicle-Stimulating Hormone Rise in Normal Ovulatory Women
Nancy A. Klein, Brenda S. Houmard, Karl R. Hansen, Teresa K. Woodruff, Patrick M. Sluss, William J. Bremner and Michael R. Soules

Previous studies have reported that the monotropic rise in FSH in older women is associated with decreased inhibin B and/or A levels and increased levels of activin A. Whereas most investigators have found decreased follicular-phase inhibin B, the roles of inhibin A and activin A as modulators of the FSH rise are unclear. The objectives of this study were to determine whether deficiencies in circulating levels of inhibin A, inhibin B, and/or activin A exist during the intercycle interval in ovulatory older (age, 40–45 yr; n = 16), compared with younger women (age, 20–25 yr; n = 13). Blood samples were obtained daily throughout one menstrual cycle and the follicular phase of the subsequent cycle and were analyzed for LH, FSH, estradiol, inhibin A and B, and activin A. Despite significant FSH elevation, no deficiencies in inhibin A, activin A, or estradiol were detected in older subjects. In fact, inhibin A was significantly higher in older participants during the intercycle phase (P = 0.01), whereas inhibin B was significantly lower. Thus, the monotropic rise in FSH does not appear to result from changes in inhibin A or activin A, supporting the concept that inhibin B plays a critical role in mediating the FSH rise in older women.