FSH Levels: Follicle-Stimulating Hormone Function in Fertility and Menopause

What is FSH?

Follicle-stimulating hormone (FSH) is a hormone produced by a small gland inside your head called the anterior pituitary. FSH is released in response to another hormone called gonadotropin-releasing hormone, which is secreted by your hypothalamus.

Your pituitary gland also releases luteinizing hormone (LH). Both FSH and LH are gonadotropin hormones, which means that they tell the ovaries and testicles to secrete other hormones such as progesterone, estradiol, testosterone, and inhibin. These hormones play important roles in human reproduction and fertility.

What does FSH do? The functions of follicle-stimulating hormone explained

FSH has important roles in human sexual development and function. FSH levels are usually low in children and increase once puberty starts. During puberty, FSH levels rise, stimulating ovaries to secrete estrogen and testicles to secrete testosterone. These initial changes are responsible for the multiple effects that puberty has on the body.

FSH levels vary throughout the menstrual cycle. FSH plays a vital role in stimulating follicular development so that ovulation can occur. 

The follicular phase of the menstrual cycle is the stage between the first day of menstruation and ovulation. During this phase, FSH stimulates several follicles inside the ovaries, which causes them to start maturing. FSH levels are highest during the first week of this phase.

The growing follicles compete for dominance until a single follicle becomes dominant. At that point, this follicle begins to secrete estradiol and inhibin, which suppress FSH secretion. The highest level of FSH coincides with an LH peak, which leads to ovulation.

The drop in FSH levels causes the remaining follicles — which didn’t become dominant — to degrade and die. FSH remains low during the luteal phase, which prevents new follicles from maturing.

Since FSH plays such a vital role in follicle development and ovulation, it’s easy to see why abnormal levels of FSH are often associated with infertility. FSH can also play a role in alterations of the menstrual cycle, low male sex drive, and delayed or early onset of puberty.

FSH also stimulates sperm production. Unlike women’s FSH levels, which fluctuate according to the stages of the menstrual cycle, men’s FSH levels tend to remain stable.

Difference between FSH and LH levels

The roles of FSH and LH are different, but they work together to regulate fertility. These two hormones are so vital that a normal menstrual cycle can be divided into distinct phases, using an FSH and LH surge as reference.

Similarly to FSH, LH is a glycoprotein hormone secreted by the anterior pituitary. If you’re undergoing fertility testing, your doctor might order both FSH and LH blood tests. These tests can be used to:

• Determine what is causing your fertility problems
• Discover abnormalities in your ovarian function
• Establish the cause for irregular or absent menstrual cycles
• Confirm whether you’re entering perimenopause or menopause (although FSH testing isn’t commonly carried out in healthy perimenopausal or menopausal women)

Doctors might also use the LH-to-FSH ratio to diagnose polycystic ovary syndrome (PCOS), a relatively common hormonal disorder. The diagnostic criteria for PCOS is a 3:1 LH-to-FSH ratio (unless the abnormal ratio can be explained by a different condition).

What is the FSH blood test?

To measure your FSH levels, your doctor will use a thin needle to take a blood sample from a vein in your arm and send it to a lab for processing. Giving a blood sample is a quick procedure that only takes a few minutes. It might hurt a little, and there might be a small bruise where the needle was inserted.

Your doctor may schedule this test during a specific point in your menstrual cycle. It’s also possible to determine FSH levels through a urine test. In this instance, your doctor may ask you to collect a single sample of urine or multiple samples throughout a 24-hour period.

High levels of FSH could indicate:

• Primary ovarian insufficiency or premature ovarian failure
• PCOS
• The presence of an ovarian tumor
• Turner syndrome, a genetic disorder that affects female development
• Perimenopause or menopause

Low FSH levels, on the other hand, could indicate:

• Malnutrition or severely low weight
• Your pituitary gland or hypothalamus isn’t working correctly

Fertility in women

Reference values for FSH are measured in international units per liter (IU/L) and help determine the range of normal function. Women’s FSH values will vary depending on their age and what stage of sexual maturity they have reached.

Although reference values can vary, women with an FSH level higher than 10 IU/L on cycle day three may have a diminished ovarian reserve, which can lead to fertility problems. In these cases, the pituitary gland releases more FSH to try to recruit additional follicles for ovulation. 

It’s important to discuss your test results with your doctor to determine the underlying causes for your FSH results and whether you need treatment.

Fertility in men

Abnormal FSH levels in men could also signal a fertility problem. 

Since LH and FSH play a role in stimulating testosterone and sperm production, any condition that lowers the levels of these two hormones, such as pituitary tumors, can decrease sperm production and testosterone levels.

High levels of FSH can be due to:

• Testicle damage caused by radiation, chemotherapy, certain infections, or alcohol abuse
• Klinefelter syndrome, a genetic condition caused by having an extra X chromosome that hinders male sexual development and usually causes infertility
• Other chromosomal disorders that alter testicular function
• Cryptorchidism or undescended testicles

The bottom line

Follicle-stimulating hormone is released by your anterior pituitary gland and plays a key role in the regulation of your reproductive function. FSH works alongside the luteinizing hormone to promote follicular growth inside your ovaries during each menstrual cycle, culminating in ovulation. 

Along with other hormones, FSH and LH are responsible for normal menstrual cycles. Some women get an FSH test to determine their levels of this hormone during fertility testing. Excessively low or high levels of FSH could indicate different causes of infertility, such as entering menopause or having a decreased ovarian reserve.

If you have received an abnormal FSH test result, your doctor will discuss these tests and their significance with you. Depending on the underlying cause of the issue, different treatments could help correct this imbalance.