Special Precautions/Comments:

Ideally sample should be taken day 2 – 7 of the cycle (follicular phase).

Exogenous oestrogen or testosterone can suppress the hypothalamic-pituitary-gonadal axis and affect interpretation.

Additional Information:

Follicular stimulating hormone (FSH) is a gonadotrophin secreted from the anterior pituitary in a pulsatile manner in response to secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus. FSH works in conjunction with luteinising hormone (LH) to regulate growth and function of the gonads (ovaries and testes). In females, FSH stimulates the growth and maturation of immature follicles, selecting one dominant follicle ready for ovulation. FSH also stimulates the production of estradiol (E2). FSH starts to rise in the follicular phase, reaching its peak mid-cycle (ovulation). In the luteal phase, FSH levels decrease. In males, FSH stimulates spermatogenesis in Sertoli cells.  

FSH can be measured alongside LH and E2 or testosterone to investigate the function of the hypothalamic-pituitary-gonadal axis. Measurement of prolactin and progesterone can also be useful. FSH can be measured in the investigation of amenorrhoea, erectile dysfunction, infertility, premature menopause and polycystic ovarian syndrome (PCOS). It may also be measured to assess pituitary function.  

Elevated FSH can be seen in primary gonadal failure and in the menopause. Measurement of FSH to assess perimenopause is only recommended in women <40 years or 40-45 years with symptoms. FSH can be measured in women >45 years if they have atypical symptoms.  

Decreased FSH can be seen in hypogonadotrophic hypogonadism which may be caused by a hypothalamic issue (such as GnRH deficiency). It can also be seen in hypopituitarism, where there is no production of 1 or more of the pituitary hormones. Decreased FSH is also seen in men with azoospermia. In PCOS, an elevated LH:FSH may be seen, but this is not always the case.

FSH is suppressed in the presence of high levels of oestrogen. This can be seen in pregnancy and in use of exogenous oestrogens, including the oral contraceptive pill and hormone replacement therapy (HRT). Similarly, FSH can be suppressed by high levels of testosterone such as in testosterone replacement therapy. Finally, FSH can be suppressed by high levels of prolactin (hyperprolactinaemia).