Follicle stimulating hormone (FSH) is produced by the small gland at the base of the brain called the pituitary gland. FSH is known mainly for its role in regulating the menstrual cycle in women, but is also found in men. In men, FSH stimulates sperm production.

During a normal menstrual cycle, FSH level rises during the first half of the cycle to stimulate the growth of egg follicles. This is the primary function of FSH, hence the name “follicle stimulating hormone”. Once the egg follicle matures and bursts to release the egg during ovulation, the ruptured follicle forms a structure called the corpus luteum that’s primary function is the production of progesterone. The surge of progesterone inhibits the release of FSH during the second half of the menstrual cycle. As the corpus luteum gradually breaks down, progesterone level falls, FSH level rises and the next menstrual cycle begins again.

FSH also stimulates the production of oestradiol by the egg follicles in the ovaries. Oestradiol and FSH form a so-called “negative feedback loop”, which means that high levels of oestradiol will inhibit the production of FSH. As a woman ages and approaches menopause, the ability of her ovaries to produce oestradiol decreases and FSH level will consequently increase to stimulate the ovaries to work harder at producing oestradiol. High FSH level is therefore often the first sign that menopause is approaching and appears before the decrease in oestradiol levels. After menopause, FSH gradually decreases as women become older.

It is important to check FSH levels when a woman is having difficulty getting pregnant, or having irregular periods or are experiencing symptoms such as hot flushes, night sweats, difficulty sleeping, all suggestive of possible menopause.

A FSH test can also be ordered for a man if his partner is experiencing difficulty getting pregnant or if he is experience problems such as low sperm count, low muscle mass or decreased sex drive.

In both men and women, a FSH test can be ordered together with a number of other hormone tests such as luteinising hormone (LH), thyroid stimulating hormone (TSH), prolactin and growth hormone (GH) to investigate the function of the pituitary gland, in particular the anterior (front) part of it which secretes these hormones. When the pituitary gland is not functioning correctly, this can lead to vague symptoms such as fatigue, weakness.

In short, it is useful to analyse FSH levels in the blood to:

• Determine the menopausal status of a woman;

• Investigate the possible reasons fertility issues in both men and women;

• Investigate suspected pituitary disorders.

The reference range for FSH levels in the blood can be different depending on the laboratory and technique used. Doctors usually also take into account a number of factors when evaluating FSH values.

High FSH levels in the blood may be associated with:

• Perimenopause or menopause

• Ovarian failure

• Test taken in mid-cycle phase of the menstrual cycle

• Pituitary disorders

• Low sperm count in men

• Chronic overconsumption of alcohol

Low FSH levels in the blood may be associated with:

• Polycystic ovary syndrome (PCOS)

• Test taken in the luteal phase of the menstrual cycle

• Difficulty getting pregnant

• Pituitary disorders

• Low sperm count in men

• Prolonged fasting

• High levels of stress

• Use of hormonal contraceptives

FSH levels fluctuate during the menstrual cycle. It is recommended for menstruating women to always take the test on day 3 in their menstrual cycle. For women who do not have menstrual cycles, and men, the test can be taken at any time.

Many hormonal contraceptives contain synthetic oestrogen which can interfere with the analysis of FSH. It is therefore not recommended to take the test if you are taking hormonal contraceptives.

A single increased FSH value is not enough to diagnose menopause. Due to normal fluctuations, repeat testing is often needed and clinical presentation also needs to be taken into account.

Janet E. Hall, M.D. Endocrinology of the Menopause. Endocrinol Metab Clin North Am. 2015 Sep; 44(3): 485–496.