What is follicle-stimulating hormone (FSH)?
Follicle-stimulating hormone (FSH) is a hormone your pituitary gland makes and releases that plays a role in sexual development and reproduction. It affects the function of ovaries and testicl*s.
Despite its name, follicle-stimulating hormone doesn’t directly affect your hair follicles or hair growth. A special group of hormones called androgens affects hair growth. FSH got its name due to its effect on ovarian follicles, which are small sacs filled with fluid that contain egg cells in the ovaries.
Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.
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What is the function of follicle-stimulating hormone (FSH)?
Follicle-stimulating hormone is an important hormone that has a few different functions depending on the type of sexual organs (gonads) you have and your age.
FSH function in fetal development
In the second and third trimesters of pregnancy (week 13 to the end of week 26 and week 27 to the end of the pregnancy, respectively), the fetus’s pituitary gland releases FSH and luteinizing hormone (LH).
These hormone levels peak midpregnancy as the first ovarian follicle or seminiferous tubule (coiled tubules within the testes) mature in the fetus.
FSH function during puberty
FSH levels are normally low in children. As puberty approaches (usually between ages 10 and 14), the hypothalamus produces gonadotropin-releasing hormone (GnRH), which triggers FSH and LH. This begins the changes toward sexual maturity and development.
In boys or children assigned male at birth (AMAB), FSH and LH work together to trigger their testes to begin producing testosterone. This is the hormone responsible for the physical changes of puberty (such as body hair growth and voice deepening) and the production of sperm.
In girls or children assigned female at birth (AFAB), FSH and LH trigger their ovaries to begin producing estrogen. This hormone is responsible for physical changes of puberty, such as breast development and menstruation.
FSH function in menstruating people assigned female at birth (AFAB)
For women or people AFAB who are menstruating, FSH’s main function is to help regulate the menstrual cycle. Specifically, FSH stimulates follicles on the ovary to grow and prepare the eggs for ovulation. As the follicles increase in size, they begin to release estrogen and a low level of progesterone into your blood.
Ovulation is a phase in the menstrual cycle. It occurs on about day 14 of a 28-day menstrual cycle. Specifically, ovulation is the release of the egg (ovum) from an ovary.
Each month, between days six and 14 of the menstrual cycle, FSH causes follicles in one of the ovaries to begin to mature. However, during days 10 to 14, only one of the developing follicles forms a fully mature egg. At about day 14 in the menstrual cycle, a sudden surge in LH causes the mature follicle to rupture and release its egg (ovulation).
After ovulation, the ruptured follicle forms a corpus luteum (a temporary endocrine gland) that produces high levels of progesterone. Progesterone blocks the release of FSH and helps prepare the uterine lining for pregnancy.
The egg begins its five-day travel through the fallopian tube to the uterus. At this point, if a sperm successfully fertilizes the egg, it results in pregnancy. If no fertilization occurs, towards the end of the cycle the corpus luteum breaks down. Progesterone production then decreases and the next menstrual cycle begins — you get your period — when FSH levels start to rise again.
FSH function in people assigned male at birth (AMAB)
For men or people AMAB, FSH stimulates sperm production. In collaboration with testosterone inside the testes, which is triggered by LH, FSH also sustains sperm production.
What triggers FSH production?
A complex feedback system of hormones called the hypothalamic-pituitary-gonadal axis regulates your FSH levels. If any of the glands or organs in this system aren’t working properly, it leads to abnormal levels of hormones involved in the system.
To start, your hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates your pituitary gland to release follicle-stimulating hormone (FSH) as well as luteinizing hormone (LH) — the gonadotropins. Your hypothalamus releases GnRH in pulses (small, short bursts), with low-pulse frequencies stimulating more FSH production and high-pulse frequencies triggering more LH production.
The released FSH and LH travel through your bloodstream and then bind to receptors in the testes or ovaries (the gonads). This is how FSH, along with LH, can control the functions of the testes and ovaries.
In return, hormones that your testes or ovaries release control the amount of GnRH your hypothalamus makes, starting the cycle over again. However, different amounts of some hormones in this system affect the release of other hormones differently.
For example, in people AFAB, estrogen affects levels of FSH release in different ways. Chronic (long-term) estrogen exposure inhibits FSH release, whereas rising estrogen levels, which normally occur before ovulation, trigger your pituitary gland to increase levels of gonadotropins (FSH and LH). Progesterone slows the frequency at which your hypothalamus releases GnRH but enhances gonadotropin responses to GnRH.
In people AMAB, the hormone inhibin B, which cells in the testes release, blocks FSH secretion as part of the feedback loop.
Again, any disruption or issue in this hormone release chain causes a lack of sex hormones. This prevents normal sexual development in children and normal function of the testicl*s or ovaries in adults.
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What are normal FSH levels?
Normal follicle-stimulating hormone (FSH) levels vary based on your age and sex organs. It’s also important to remember that normal ranges for FSH levels can vary from lab to lab. Always reference the lab’s normal range on your blood test report. If you have any questions about your results, be sure to ask your healthcare provider.
Normal FSH levels for men or people assigned male at birth (AMAB)
In general, normal FSH levels for men or people AMAB include:
- Before puberty: 0 to 5.0 milli-international units per milliliter (mIU/mL).
- During puberty: 0.3 to 10.0 mIU/mL.
- Adult: 1.5 to 12.4 mIU/mL.
Normal FSH levels in women or people assigned female at birth (AFAB)
In general, normal FSH levels for women or people AFAB include:
- Before puberty: 0 to 4.0 mIU/mL.
- During puberty: 0.3 to 10.0 mIU/mL.
- Menstruating people: 4.7 to 21.5 mIU/mL.
- After menopause: 25.8 to 134.8 mIU/mL.
What is a normal FSH level to get pregnant?
The timing for FSH testing for determining aspects of fertility is essential. For menstruating people, FSH levels vary from day to day throughout the menstrual cycle. For basic fertility testing, you need to have the FSH blood test on day 3 of your menstrual cycle (day 1 is the day your period begins).
A study on day 3 FSH levels and in vitro fertilization (IVF) outcomes showed that people with day 3 FSH levels lower than 15 milli-international units per milliliter (mIU/mL) had a better chance of becoming pregnant with an IVF attempt when compared to people with FSH levels between 15 mIU/ml and 24.9 mIU/ml. For people with FSH levels over 25 mIU/ml, pregnancy rates per attempt were even lower.
It’s important to remember that several factors affect fertility — your FSH level isn’t the only determining factor. If you have questions about your fertility and becoming pregnant, ask your healthcare provider or a fertility specialist.
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What conditions are associated with high FSH levels?
Most often, higher-than-normal levels of follicle-stimulating hormone (FSH) are a sign of an issue in the ovaries or testes (gonads).
If your gonads can’t create enough estrogen, testosterone and/or inhibin, the correct feedback loop of FSH production from your pituitary gland is lost and the levels of both FSH and LH rise. This condition is called hypergonadotropic-hypogonadism, or primary hypogonadism, and is associated with primary ovarian insufficiency (POI) or testicular failure.
Primary hypogonadism can be congenital (you’re born with it) or result from other conditions (acquired).
Congenital forms of primary hypogonadism include:
- Klinefelter syndrome in people AMAB.
- Turner syndrome in people AFAB.
- Androgen insensitivity syndrome.
- Certain enzyme deficiencies (very rare).
Conditions and situations that can cause acquired primary hypogonadism include:
- Gonadal toxins, such as radiation or chemotherapy.
- Damage or injury to the ovaries or testes.
- Sex hormone decline with aging.
- Certain autoimmune conditions.
- Infections such as mumps.
In very rare cases, issues with the pituitary gland in people AFAB can raise FSH levels. This overwhelms the normal feedback loop and can sometimes cause ovarian hyperstimulation syndrome, which causes enlarged ovaries and a potentially dangerous accumulation of fluid in the abdomen.
People AFAB naturally experience elevated FSH levels during menopause. This is normal.
For children, higher levels of FSH and LH than expected based on age — in addition to the development of secondary sexual characteristics — are an indication of precocious (early) puberty. This is much more common in children AFAB than in children AMAB. Puberty is considered early if it starts before age 9 in children AFAB and before age 10 in children AMAB.
What conditions are associated with low FSH levels?
Lower-than-normal FSH levels usually lead to incomplete development during puberty in children. They lead to poor ovarian or testicular function in adults, which can lead to infertility. This condition is called hypogonadotropic-hypogonadism, and it’s usually caused by issues with your pituitary gland or hypothalamus.
Pituitary gland issues that can cause low FSH levels
Hypopituitarism is a rare condition in which there’s a lack (deficiency) of one, multiple or all of the hormones your pituitary gland makes. Thus, hypopituitarism could potentially result in a lack of FSH.
Although it’s rare, many conditions and situations can cause hypopituitarism. In general, these three main factors can cause hypopituitarism:
- Something is putting pressure on your pituitary gland or hypothalamus.
- There’s damage to your pituitary or hypothalamus.
- You have a rare condition that can cause hypopituitarism.
One of the most common causes of primary hypopituitarism is a pituitary adenoma. If the adenoma is of a certain size or is growing, it can put pressure on your pituitary gland or block blood flow to it.
Hypothalamus issues that can cause low FSH levels.
Kallmann syndrome is an inherited condition in which your hypothalamus doesn’t make enough gonadotropin-releasing hormone (GnRH). Without enough GnRH, you have low levels of FSH and low levels of sex hormones. Without treatment, a person with Kallmann syndrome won’t enter puberty and won’t be able to have biological children.
What are the symptoms of abnormal FSH levels?
Except for abnormally high levels of FSH in kids, which can cause early puberty, abnormal levels of FSH — whether too high or too low — usually point to hypogonadism. Hypogonadism occurs when sex glands, or gonads (either testes or ovaries), produce few sex hormones, if any.
Lower-than-normal FSH levels due to an issue with your hypothalamus or pituitary gland directly cause hypogonadism. But higher-than-normal FSH levels are usually a sign or side effect of hypogonadism when an issue originates in the gonads.
This is because a lack of sex hormone production by the gonads (often due to some type of damage) causes your pituitary gland to release excess FSH to try to stimulate more sex hormone production. In this case, the FSH doesn’t have its normal effect because there’s something wrong with the tissue in the gonads that normally makes sex hormones.
Symptoms of hypogonadism vary depending on your age and sex.
Symptoms of hypogonadism in newborns can include:
- An abnormally small penis (micropenis).
- Undescended testicl*s (cryptorchidism).
Symptoms of hypogonadism in children can include:
- Absent breast development in girls or children AFAB during puberty.
- No menstruation (amenorrhea) or delayed menstruation.
- Absent testicular enlargement in boys or children AMAB.
- Lack of growth spurt during puberty.
Symptoms of hypogonadism in men or adults AMAB include:
- Loss of interest in sex.
- Fatigue.
- Infertility, likely due to a lack of sperm (azoospermia).
- Erectile dysfunction.
- Breast enlargement (gynecomastia).
- Loss of muscle tone.
- Having less facial or body hair.
Symptoms of hypogonadism in women or adults AFAB include:
- Loss of interest in sex.
- Fatigue.
- Infertility, likely due to issues with ovulation.
- Hot flashes.
- Irregular menstruation or no menstrual periods.
- Losing pubic hair.
What test measures FSH levels?
Your healthcare provider can check your follicle-stimulating hormone (FSH) levels through a blood test. It involves using a needle to draw a blood sample from a vein in your arm.
Providers may order FSH blood tests for several reasons, and they may order additional hormone blood tests to help diagnose or rule out conditions, including:
- Luteinizing hormone (LH).
- Testosterone.
- Estradiol and/or progesterone.
In general, these tests can help:
- Determine the cause of infertility.
- Diagnose conditions associated with dysfunction of the ovaries or testicl*s.
- Aid in the diagnosis of pituitary or hypothalamus conditions, which can affect FSH production.
More specifically, for people AFAB, FSH blood tests can provide information for:
- The investigation of menstrual irregularities (irregular periods).
- Predicting when or if they’re entering menopause.
For people AMAB, FSH blood tests can help determine the reason for a low sperm count.
For children, providers use FSH and LH blood tests to help diagnose delayed or early puberty.
When should I call my doctor about my FSH levels?
If you’re experiencing symptoms of hypogonadism, contact your healthcare provider.
If your child is entering puberty earlier or later than expected, contact their provider. They may run some simple blood tests to see if there’s an issue with their FSH levels.
If you’re having difficulty becoming pregnant, talk you your provider or a fertility specialist. While many factors can contribute to infertility, your FSH levels could be one of them.
A note from Cleveland Clinic
Follicle-stimulating hormone (FSH) is an important hormone that has a powerful effect on sexual development and fertility. Since FSH is part of a complex web of other hormones and glands, it’s not uncommon to have irregular FSH levels. If you or your child is experiencing symptoms related to sexual development and/or fertility, talk to your healthcare provider. They’re available to help you.
