Follicle stimulating hormoneFSH
A pituitary hormone that drives egg development and sperm production.
Follicle stimulating hormone is released by the pituitary gland at the base of the brain. In women it stimulates the ovarian follicles that contain eggs; in men it supports sperm production in the testes.
It matters because it reports on the feedback loop rather than the gland below it. When the ovaries or testes produce less, the pituitary pushes harder and FSH rises, which is why a raised FSH is one of the markers used when assessing the menopausal transition or reduced testicular function.
In practice FSH varies markedly across the menstrual cycle, so the day it is taken changes its meaning entirely, and it is read together with LH and the sex hormones themselves.