FSH is made by the pituitary gland, a small gland at the base of the brain. Early in each menstrual cycle, it signals the ovaries to start growing a group of follicles, each containing an egg. In men, FSH supports sperm production in the testes.
Why it's tested
An FSH blood test, usually done on day 2 or 3 of the period, helps assess how hard the body is working to stimulate the ovaries. A higher-than-expected FSH can suggest the ovaries are responding less easily — one of several markers doctors read together (alongside AMH and an ultrasound follicle count) when assessing ovarian reserve. One number alone never tells the whole story.
In treatment
Injectable medicines used in IVF stimulation are forms of FSH — given so that several follicles grow in one cycle instead of the usual single follicle.