Inhibin acts primarily on the pituitary to suppress the production and secretion
of FSH, and can also suppress the secretion of LH. Levels of inhibin fall during the ovulatory window, leading to the LH and FSH surges, respectively. Evidences suggest hCG acts directly on the hypothalamus to block the production of GnRH, thereby reducing production of both LH and FSH. Activin stimulates the production and/or release of the gonadotropins. Testosterone and progesterone can both block the production and release of FSH and LH, but these are steroids, not proteins.