FSH (follicle-stimulating hormone) is a pituitary hormone that drives ovarian follicles to grow.
What FSH is and what it does
FSH stands for follicle-stimulating hormone. It is released by the pituitary gland, a small gland in the brain that controls many of the body’s hormones. As its name suggests, the main job of FSH is to stimulate follicles in the ovary to grow each menstrual cycle. These growing follicles are what then help an egg mature and get ready to be released at ovulation. Without this balanced signal, the process of maturing an egg can be disrupted, and this is one reason FSH is often mentioned when discussing the menstrual cycle and fertility.
How FSH works with other hormones
FSH does not act on its own. It works in tandem with LH (luteinising hormone), and the balance between these two hormones governs ovulation. FSH and LH levels rise and fall at different points in the menstrual cycle, and this pattern helps coordinate follicle growth and the release of an egg. As a follicle grows, it also produces estrogen, which feeds back to the brain to adjust how much FSH is released next. This back-and-forth system is what keeps the cycle regular in most situations. To understand how these hormones work together, see the hormones page.
Why it matters for PCOS
In some women with PCOS the ratio of LH to FSH tends to run high, meaning LH is relatively high compared with FSH. This pattern is sometimes linked to irregular ovulation, although it is not present in everyone with PCOS and is not a confirming test for diagnosis. The link between these hormones and symptoms is explained further on the what is PCOS and PCOS symptoms pages.
When to talk to a doctor
An FSH result is interpreted in context, not on its own. It is read alongside LH, oestradiol, symptoms, and when in the cycle the sample was taken, because the timing of the test can affect the value obtained. Hormone tests like this are often taken in the early part of the menstrual cycle. Your doctor is best placed to advise the right timing for a test and to explain what your result means.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.