Follicle Stimulating Hormone (FSH)

The follicle stimulating hormone (FSH) is produced in the pituitary gland in the brain. FSH plays an important role in your monthly cycle because it allows your eggs to grow. In your ovaries, there are thousands of immature eggs contained in follicles filled with fluid. They are at different stages of development, and most of them are dormant. All these follicles form your ovarian reserve, also called the egg reserve. The follicles that are suitable for further development have FSH receptors, so they react to the FSH and continue to grow.

About 15–20 follicles (depending on the size of your ovarian reserve) grow in the first half of the cycle. They produce estrogen as a result of FSH stimulation, which in turn, inhibits FSH formation. Thus, less FSH is available in the second week of the cycle, and fewer and fewer of the original 15–20 follicles continue to grow.

The follicle that responds best to FSH stimulation produces more estrogen than any other follicle. This follicle is called the “dominant follicle.” It holds the oocyte that is ejected from the ovary during ovulation. In some cases, there are also two dominant follicles. If they are fertilized after ovulation, they grow into twins.

It is important to know that the FSH level changes during your cycle. In the first 4–5 days, it is stable at a moderately high level. It then decreases due to the estrogen formation of the follicles, only to rise sharply shortly before ovulation. If you want to find out if your FSH value is within the standard range, it is important to take the test on the second or third cycle day. Later on, having your FSH measured doesn’t make much sense.


Menstrual Cycle Estrogen FSH follicle stimulating hormone luteinizing hormone

Why is FSH Measured?

FSH is measured to provide information about the following processes in the female body:

High Levels of Follicle Stimulating Hormone

A high FSH value can be caused by the following causes, among others:

  • Ovarian tumors
  • Post-ovariectomy (ovarian removal) conditions
  • The onset of menopause
  • Polycystic Ovary Syndrome (PCOS)
  • Turner syndrome or monosomy X (one functional X chromosome instead of two)

Low Follicle Stimulating Hormone Level

A lack of FSH can have different causes. These include, among other things, a change in the pituitary gland or genetic information. Blood tests, computed tomography, or a stimulation test can be used to determine where the cause of the FSH deficiency lies.

If too little FSH is formed, ovulation remains or takes place only very irregularly, which makes it difficult to become pregnant.

A low FSH value can also be caused by, among other things,

  • Hormonal contraception (the pill, hormonal IUD, etc.)
  • Stress
  • Anorexia
  • A subfunction of the pituitary gland
  • Tumors of the pituitary gland

FSH in Reproductive Medicine

Remember the beginning of this article? Due to the interaction of FSH and estrogen in the second week of the cycle, fewer eggs continue to grow. That is precisely where reproductive medicine comes in. As part of in vitro fertilization (IVF), eggs are fertilized outside the body. The woman is given FSH so that she produces several eggs. Of the original 15–20 oocytes, it is not one dominant follicle that matures but several—ideally five to ten. These follicles can be collected from the ovary during IVF and fertilized with the partner’s or a donor’s sperm.

Depending on the size of your ovarian reserve, more or less FSH is administered during IVF. The best way to assess the size of your ovarian reserve is to have your anti-Mullerian hormone (AMH) measured. That is why AMH is measured in most fertility centers before treatment begins. If a woman has a low ovarian reserve, or if she is already in her menopausal transition, a higher dose of FSH is necessary.