The role of gonadotropins in the follicular phase.

Folliculogenesis in humans is a lengthy process that involves several regulators. Pituitary gonadotropins play crucial roles in the late stages, particularly in the last 15 days of follicle maturation. During the intercycle rise of follicle-stimulating hormone (FSH), selection of the dominant follicle takes place. This is a complex process that also involves locally produced substances. At the same time, luteinizing hormone (LH) stimulates the synthesis of androgens, which serve as the substrate for the production of estrogens. During the second half of the follicular phase, the follicle becomes dependent on LH. Induction of multiple follicular development by exogenous FSH results in a marked suppression of endogenous LH. For normal follicle maturation, both an LH threshold and an LH ceiling have been considered. In the context of an in vitro fertilization program, application of protocols for ovarian stimulation that will prevent the marked suppression of endogenous LH secretion might provide a better approach to treatment optimization.