FSH (Follicle-Stimulating Hormone)

Follicle-stimulating hormone (FSH) is a hormone that plays a key role in the reproductive function of both females and males. It is produced by the pituitary gland in response to signaling from the hypothalamus, and acts on specific cells in the ovaries and testes. In women, FSH allows for an egg to be matured and ovulated each month, while in men it facilitates sperm production.

In some people, FSH levels may be too high, or too low. This can be due to genetic causes, or acquired causes. Conditions that cause abnormal levels of FSH can present in different ways, but some of the most common symptoms of both high and low FSH are irregular or absent periods, a low sperm count, and male and female infertility.

Read on to learn about FSH, its function, how FSH levels can be tested, and what it means if they are found to be abnormal.

Follicle stimulating hormone, commonly abbreviated to FSH, belongs to the ‘gonadotropin’ group of hormones. It is important in both male and female reproductive function, acting on granulosa cells in the female ovaries, and on Sertoli cells in the male testes. FSH levels in the body are regulated by a well-documented endocrine signaling pathway called the hypothalamic-pituitary-gonadal (HPG) axis. 1 2

The pituitary gland, a small endocrine gland that sits at the base of the brain, lies at the center of the HPG axis. FSH is produced by the gonadotroph cells in the front, or anterior, half of the pituitary gland. In addition to FSH, the pituitary gland produces many other hormones vital to regulating different processes in the body. Some of these include luteinizing hormone (LH), prolactin, thyroid-stimulating hormone (TSH) and growth hormone (GH). 3

The hypothalamus, a region of the brain, acts as a regulator of the pituitary secretion of FSH. It sends pulses of gonadotropin-releasing hormone (GnRH) to the pituitary, which signals when to produce FSH and release it into the bloodstream, and how much FSH should be produced. 1

Once FSH enters the bloodstream it travels to the gonads, or reproductive organs, where it acts directly on its target cells.

As its name suggests, FSH stimulates the development and maturation of ovarian follicles in the female reproductive system. It plays an equally important role in the male reproductive system, making it possible for sperm to be produced.

In women, FSH acts directly on the granulosa cells in the ovaries. At the beginning of the menstrual cycle, FSH levels in the blood increase. This stimulates the maturation of one of the ovarian follicles, which contains an egg (ovum). As this happens, the follicle begins to produce estradiol, and its levels in the blood rise. FSH levels momentarily drop as estradiol levels increase. Around day 14 however, the peaking estradiol levels cause a rapid surge in FSH and LH levels. This directly results in ovulation – the release of the now mature ovum from its follicle. After ovulation, FSH levels drop in response to a rise in progesterone, and they remain low until the next menstrual cycle begins. 1 2

In men, FSH acts on the Sertoli cells, which are located in the testes. These cells are responsible for the production of sperm. 4

Similarly to other reproductive hormones such as estradiol, LH or progesterone, FSH levels in women fluctuate depending on the stage of the menstrual cycle. The National Health Service (NHS) in the United Kingdom cites the following as normal FSH ranges for women: 5

• Days 1-14 (Follicular phase): 3.3-11.3mlU/mL
• Day ~14 (Ovulation): 5.2-20.4 mlU/mL
• Days 14-28 (Luteal phase): 1.8-8.2 mlU/mL

FSH levels are stable in men, and do not fluctuate after puberty. The suggested FSH normal range for men is between 1.6 and 11.0 mlU/mL. 5

In some people, FSH is found to be low. Conditions affecting the pituitary gland and/or the hypothalamus are usually the culprit of low FSH levels.

The state of having low blood FSH levels and low sex hormone levels is called hypogonadotropic hypogonadism. In individuals with hypogonadotropic hypogonadism, the pituitary gland does not produce the required amounts of FSH and LH. Because of this, estradiol and inhibin levels are low in women, and follicle maturation is impaired. Meanwhile, low FSH in males results in less sperm being produced. This can cause various issues, most notably absent or irregular periods in women, and infertility in both sexes. If hypogonadotropic hypogonadism affects children or adolescents, it may result in delayed puberty, underdeveloped reproductive organs in both sexes, and absent periods in girls. 3 6

Causes of hypogonadotropic hypogonadism include conditions and syndromes that you can be born with, and ones that can occur later in life. A very large number of genetic mutations have been found to cause low FSH. Because of this, the spectrum of symptoms that somebody with hypogonadotropic hypogonadism can experience is also very broad. Damage to the central nervous system or brain tumors can cause hypogonadotropic hypogonadism later in life. 6

Low FSH and sex hormone levels can be treated with hormone supplements. However, if you are worried about having low FSH, you should first consult your physician. It may also be useful to get an FSH test.

Sometimes, FSH levels are found to be higher than the norm. This can be accompanied by either low, normal, or high sex hormone levels.

The state of having high blood FSH levels and low sex hormone levels is called hypergonadotropic hypogonadism. In certain conditions or situations, the gonads fail to produce enough sex hormones. When this happens, the HPG axis sends feedback to the hypothalamus and the pituitary, informing it that more FSH and LH is needed to normalize their levels. In response to this, FSH levels rise above the normal range, which can be detected by a blood test. 3

Many different conditions and syndromes can cause hypergonadotropic hypogonadism. These range from chromosomal and genetic mutations and mutations in the structure of FSH and LH themselves, to Turner Syndrome and Klinefelter Syndrome. Onset can also occur later in life, though this is rarer. Absent or irregular periods and infertility are common presenting complaints. 6

Primary ovarian insufficiency (POI), also known as premature ovarian failure, is a frequently encountered cause of high FSH levels. In POI, a woman’s ovaries stop functioning normally earlier than they are intended to. When this happens, they only release eggs intermittently, or stop releasing them altogether. This correlates with a decrease in the reproductive hormones produced by the ovaries. Irregular or absent periods, symptoms of menopause and infertility in women under the age of 40 are the major symptoms to be mindful of. If POI is suspected, your doctor will perform two FSH tests, four weeks apart from each other. If both tests show elevated FSH levels, a diagnosis of POI can be made. 7

Pituitary adenomas are a rarer cause of high FSH levels. A functioning pituitary adenoma is a type of tumor that grows from the cells of the pituitary gland, and contributes to the production of more pituitary hormones. If gonadotroph cells happen to be affected by a pituitary adenoma, more FSH is produced and its levels become elevated. 1 Pituitary adenomas are benign, and therefore not cancerous.

The treatment of high FSH levels varies based on the underlying cause. If you are worried about your FSH levels, you should consult your doctor and consider an FSH test.

It is important to understand that FSH does not act in isolation from other hormones. The action of FSH is strongly linked to the action of LH, which is also produced by the pituitary gland, and also acts on the gonads. Only through the combined action of FSH and LH on different ovarian cells can sex hormones be produced. 2 Similarly, a surge in both LH and FSH around day 14 of the menstrual cycle allows for ovulation to happen. 1

In men, the mechanisms of action of FSH and LH are more independent of one another. However, they still jointly contribute to healthy reproductive function. While FSH facilitates sperm production via the Sertoli cells, LH acts on the Leydig cells to promote testosterone production. 4

The proportion between the circulating levels of LH and FSH is a useful diagnostic tool for polycystic ovarian syndrome (PCOS). PCOS is a very common endocrinological condition in women, and can present with a wide range of bothersome symptoms. In individuals with normal gonadal function, the LH:FSH ratio is usually 1:1. However, in PCOS it is generally elevated, reaching 2:1 or as high as 3:1. 8 9

There are many reasons why you may wish to get your FSH levels checked. If you are a woman having irregular or absent periods, fertility problems, symptoms of menopause before the age of 45, or other symptoms of abnormal pituitary activity, you may wish to consider an FSH blood test. If you are a man and a doctor has diagnosed you with a low sperm count, you may also wish to consider this test to exclude HPG axis-related causes. 10

To test your FSH levels, a small sample of blood from a vein, usually one in your arm, is needed. If you are a woman, this test should ideally be performed when you are on your period; between days 1 and 4 of your menstrual cycle. 10

Q: What does FSH stand for?

A: The FSH medical abbreviation is short for follicle-stimulating hormone.

Q: What is the definition of FSH?

A: FSH is a gonadotropin – a type of hormone produced by gonadotroph cells at the front of the pituitary gland.

Q: What is the FSH function?

A: In women, FSH promotes the maturation of ovarian follicles, and consequently the production of estradiol. In men, FSH acts on Sertoli cells in the testes to promote sperm production.

Q: What does high FSH mean?

A: High FSH hormone levels usually indicate a problem in the reproductive organs, which produce insufficient sex hormones. The feedback loop between the gonads, hypothalamus, and pituitary gland is consequently disturbed. This state is called hypergonadotropic hypogonadism. Pituitary adenomas can also cause high FSH.

Q: How to lower FSH levels?

A: Diagnosing and treating the underlying medical problem causing high FSH can help restore its normal levels.

Q: What does low FSH mean?

A: Low FSH hormone levels indicate that either the hypothalamus is not producing GnRH correctly, or that the pituitary gland is not secreting sufficient FSH. Because of this, the gonads produce insufficient sex hormones. This state is called hypogonadotropic hypogonadism.

Q: When should I seek medical help?

A: If you have symptoms of gonadal or pituitary insufficiency, it may be worth consulting a doctor. Symptoms of these conditions include irregular periods, infertility, symptoms associated with low estradiol or testosterone, menopausal symptoms, and low sperm count. You may wish to consider an FSH blood test.