Follicle-stimulating hormone (FSH) is made by the pituitary gland in the brain. Control of FSH production is a complex system involving hormones produced by the gonads (ovaries or testes), the hypothalamus and the pituitary. FSH stimulates the growth and maturation of ovarian follicles (eggs) during the follicular phase of the menstrual cycle in women, which is divided into two phases, the follicular and the luteal, by a mid-cycle surge of FSH and luteinizing hormone (LH). Ovulation will occur shortly after the mid-cycle surge of hormones. During the follicular phase, FSH will initiate the production of estradiol by the follicle, and the two hormones work together in further development of the egg follicle. During the luteal phase, FSH will stimulate the production of progesterone. Both progesterone and estradiol will help the pituitary control the amount of FSH produced. FSH facilitates the ability of the ovary to respond to LH as well. At the time of menopause, the ovaries will stop functioning and FSH levels will rise. In men, FSH will stimulate the testes to produce mature sperm and will promote the production of androgen binding proteins as well. After puberty, FSH levels are relatively constant in males.