Follicle-stimulating hormone (FSH) is a hormone produced by the pituitary gland in the brain. It plays a crucial role in regulating the reproductive processes in both men and women. In women, FSH stimulates the growth of ovarian follicles, which contain eggs, and is essential for the menstrual cycle. In men, FSH is involved in the production of sperm. FSH levels fluctuate throughout a person's life and can be influenced by various factors, including age and health conditions. Measuring FSH levels can help diagnose reproductive issues, such as polycystic ovary syndrome (PCOS) in women or hypogonadism in men.