Follicle-stimulating hormone (FSH): definition
Follicle-stimulating hormone (FSH) is a hormone released by the pituitary gland in the brain. In the menstrual cycle it signals the ovaries to grow follicles, each holding an egg, and to make estrogen. FSH is highest at the start of the cycle and gives a small spike at ovulation. Levels rise a lot as the ovaries slow down near menopause.
How it works in your cycle
The hypothalamus sends GnRH pulses to the pituitary, which releases FSH and LH. FSH recruits follicles. As they make estrogen and inhibin, those feed back and lower FSH, so only the strongest follicle keeps growing. It's a self-balancing loop.
| Phase | FSH |
|---|---|
| Early follicular | Rising, recruits follicles |
| Mid to late follicular | Falls as estrogen climbs |
| Ovulation | Small spike with the LH surge |
| Luteal | Low |
What FSH tests show
- High FSH on cycle day 2 to 5: the ovaries need a stronger push, a sign of lower ovarian reserve or approaching menopause
- Persistently high FSH under 40: can point to premature ovarian insufficiency
- Low FSH: may mean the brain isn't sending signals, as in hypothalamic amenorrhea
Why it matters
FSH swings a lot in perimenopause, so a single test often can't confirm it in people over 45: doctors go by symptoms and cycle changes instead. Those years of shifting hormones are also when many people notice new mood swings, anxiety or sleep problems. FSH injections are used in fertility treatment to grow several follicles at once.
Ask a doctor about hormone testing if your periods stop for three months, become very irregular, or you have menopause symptoms before 40.
Frequently asked questions
What is a normal FSH level?
It depends on the cycle day and lab. On day 3, results under about 10 IU/L are commonly considered typical in younger adults. Your doctor will interpret it in context.
Can FSH confirm menopause?
After 45, menopause is usually diagnosed from symptoms and a year without periods. FSH is more useful under 45 when early menopause is suspected.
Sources
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Luteinizing hormone (LH): definition and meaningLuteinizing hormone (LH) is a pituitary hormone that triggers ovulation and supports progesterone. Normal levels, the LH surge and what tests show.
What is a follicle?An ovarian follicle is a small fluid-filled sac that holds and matures an egg. How follicles grow each cycle, what a dominant follicle is, and why it matters.
Menstrual cycle hormones: a simple chart of what rises and fallsMenstrual cycle hormones explained simply: how estrogen, progesterone, LH and FSH rise and fall through each phase, and how they shape mood and energy.
Ovarian reserve: definition and meaningOvarian reserve is the number of eggs left in your ovaries. How AMH and follicle counts estimate it, what it can and can't predict, and how age affects it.
Perimenopause: definition and meaningPerimenopause is the transition before menopause when hormones fluctuate and periods change, usually starting in your 40s. Signs, mood changes and timing.