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. Doctors read it together with AMH and an ultrasound follicle count to judge ovarian reserve (how many eggs are left). One number alone is never enough.
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.