FSH is crucial to fertility in both men and women. The hormone, produced by a gland in the brain, stimulates the growth of the follicles in the ovaries, which contains the developing egg, the fluid around the egg and the cells there that produce the hormones needed to support pregnancy. Without this hormone and the consequent growth of the ovarian follicle, the production of estrogen produced by the cells in the follicle would be interrupted and limited. Therefore FSH is a key component of ovulation and fertility, controlling the menstrual cycle and egg production in women.

The hormone level is important for men as well. FSH controls sperm production in men and is an indicator of sexual health and well-being. 

The FSH blood test measures the amount of follicle stimulating hormone and is a key component in the diagnosis of fertility problems. It is typically ordered in combination with other related hormones. For the evaluation of sexual function and fertility in women, the luteinizing hormone and estrogen tests are usually added to the blood panel. For the evaluation of sexual function in men, the luteinizing hormone and testosterone levels are also measured. On the flip side, when testosterone levels or estrogen levels are low, individuals may order the FSH blood test to see if low levels of FSH are to blame.