Follicle stimulating hormone

Follicle stimulating hormone is produced by the pituitary gland. It regulates the functions of both the ovaries and testes. Lack or low levels of it can cause subfertility in both men and women.

Alternative names for follicle stimulating hormone (FSH)

What is follicle stimulating hormone?

Follicle stimulating hormone (or FSH) is one of the gonadotrophic hormones; the other being luteinising hormone (or LH). Both are released by the pituitary gland into the bloodstream. Follicle stimulating hormone is one of the hormones essential to pubertal development and the function of women’s ovaries and men’s testes. In women, this hormone stimulates the growth of ovarian follicles in the ovary before the release of an egg from one follicle at ovulation. It also increases oestradiol production from the ovaries. In men, follicle stimulating hormone acts on the Sertoli cells of the testes to stimulate sperm production (spermatogenesis).

How is follicle stimulating hormone controlled?

The production and release of follicle stimulating hormone is regulated by the levels of a number of circulating hormones released by the ovaries and testes. This system is called the hypothalamic–pituitary–gonadal (HPG) axis. Gonadotrophin-releasing hormone (GnRH) is released from the hypothalamus and binds to receptors in the anterior pituitary gland to stimulate both the synthesis and release of follicle stimulating hormone (FSH) and luteinising hormone (LH). As per the definition of a hormone, the released follicle stimulating hormone is carried by the bloodstream and binds to receptors in the testes and ovaries.

In women, when FSH levels fall towards the end of the menstrual cycle. This is sensed by the hypothalamus, which produces more gonadotrophin-releasing hormone (GnRH), which in turn stimulates the pituitary gland to produce more follicle stimulating hormone (FSH) and luteinising hormone (LH), and release these into the bloodstream. The rise in follicle stimulating hormone stimulates the growth of the follicle in the ovary.

As the follicles in the ovary grow, they produce increasing amounts of oestradiol and inhibin. In turn, these hormones act on the hypothalamus and pituitary gland to reduce the release of gonadotrophin-releasing hormone and follicle stimulating hormone (negative feedback). This causes a fall in FSH, which prevents too many follicles developing each cycle. The largest follicle is less dependent on FSH for its growth and is able to continue growing even if FSH levels fall. However, smaller follicles can no longer survive as FSH levels fall due to negative feedback, enabling the emergence of a single dominant follicle.

As this follicle grows, it produces more and more oestrogen, which stimulates a surge in luteinising hormone and follicle stimulating hormone (positive) from the pituitary gland. This rise in LH leads to the release of an egg from the follicle, termed ‘ovulation’.

In summary, during each menstrual cycle, there is a rise in follicle stimulating hormone secretion in the first half of the cycle (follicular phase) that stimulates follicular growth in the ovary. After ovulation the ruptured follicle forms a corpus luteum that produces high levels of progesterone (luteal phase). Towards the end of the cycle the corpus luteum breaks down, progesterone production decreases and the next menstrual cycle begins as follicle stimulating hormone levels start to rise again.

In men, the production of follicle stimulating hormone is regulated by circulating levels of inhibin. If spermatogenesis is impaired, this will lead to less inhibin being produced by the testes. The usual action of inhibin is to inhibit FSH release from the pituitary gland. Thus, impaired spermatogenesis leads to less inhibin and thus an increase in FSH release from the pituitary gland.

Testosterone levels within the testes are also needed for spermatogenesis. This testosterone production within the testes is stimulated by the action of luteinising hormone. This ‘negative feedback’ control of testosterone on LH secretion and spermatogenesis on inhibin secretion ensures that the production of spermatogenesis and sex steroids remains steady. Excess testosterone in the body (for example after taking extra testosterone-like drugs in order to stimulate muscle growth) can result in low levels of FSH and LH and thus impair spermatogenesis.

What happens if I have too much follicle stimulating hormone?

Most often, raised levels of follicle stimulating hormone are a sign of malfunction in the ovary or testis. If the gonads fail to create enough oestrogen, testosterone and/or inhibin, due to loss of negative feedback on the pituitary gland, the levels of both follicle stimulating hormone and luteinising hormone will rise. This condition is called hypergonadotrophic-hypogonadism, and is associated with primary ovarian insufficiency or testicular failure. This is seen in conditions such as Klinefelter’s syndrome in men and Turner syndrome in women.

In women, follicle stimulating hormone levels also start to rise naturally in women in the lead up to menopause, reflecting a reduction in the function of the ovaries and decline of oestrogen and inhibin production.

There are very rare pituitary conditions that can raise the levels of follicle stimulating hormone in the bloodstream. This overwhelms the normal negative feedback loop and can (rarely) cause ovarian hyperstimulation syndrome in women. Symptoms of this include enlarging of the ovaries and a potentially dangerous accumulation of fluid in the abdomen (triggered by the rise in ovarian steroid output), which leads to pain in the pelvic area.

What happens if I have too little follicle stimulating hormone?

In women, a lack of follicle stimulating hormone leads to incomplete development at puberty and poor ovarian function (ovarian insufficiency). In this situation ovarian follicles do not grow properly and do not release an egg, thus leading to infertility. Since levels of follicle stimulating hormone in the bloodstream are low, this condition is called hypogonadotrophic hypogonadism. This is seen in a condition called Kallman’s syndrome, which is associated with a reduced sense of smell.

Sufficient follicle stimulating hormone action is also needed for full sperm production. In the case of complete absence of follicle stimulating hormone in men, lack of puberty and infertility due to lack of sperm (azoospermia) can occur. Partial follicle stimulating hormone deficiency in men can cause delayed puberty and limited sperm production (oligozoospermia), but fathering a child may still be possible. If the loss of follicle stimulating hormone occurs after puberty, there will be a similar loss of fertility.