Follicle-Stimulating Hormone (FSH) Blood Test Kit

Follicle stimulating hormone (FSH) is produced by the pituitary gland in the brain and is essential for reproductive function in both women and men. The pituitary releases FSH in response to signals from the hypothalamus, and FSH levels are regulated through a feedback loop with the sex hormones produced by the ovaries or testes. In women: FSH stimulates the growth and development of ovarian follicles—the fluid-filled sacs in the ovaries that contain eggs. During the first half of the menstrual cycle (follicular phase), FSH promotes the development of several follicles, one of which will become dominant and release a mature egg at ovulation. As follicles develop, they produce oestrogen, which provides negative feedback to reduce FSH secretion. FSH levels are highest at the beginning of the menstrual cycle, drop mid-cycle after the dominant follicle is established, and remain low in the second half (luteal phase). FSH is a key marker for assessing ovarian reserve—the quantity of remaining eggs. As women age and ovarian reserve declines, the pituitary produces more FSH in an attempt to stimulate the increasingly less responsive ovaries. Therefore, rising baseline FSH (measured on days 2-5 of the cycle) indicates diminishing ovarian reserve. An elevated baseline FSH (typically above 10-12 IU/L, though this varies by laboratory) suggests reduced fertility potential. Very high FSH (typically above 25-40 IU/L) with low oestradiol indicates menopause or premature ovarian insufficiency (POI)—the ovaries are no longer responding to stimulation. In polycystic ovary syndrome (PCOS), FSH is often normal or relatively low compared to LH (luteinising hormone). The LH: FSH ratio is frequently elevated in PCOS, which helps distinguish it from other causes of menstrual irregularity. In men: FSH acts on the Sertoli cells in the testes to support sperm production (spermatogenesis). It's essential for maintaining healthy sperm development. Low FSH in men may indicate pituitary problems; elevated FSH with low sperm counts suggests the testes are not responding normally to stimulation (primary testicular failure). Results outside the normal range may need a follow-up with your GP. For fertility assessment, FSH is often interpreted alongside other markers including LH, oestradiol, and anti-Müllerian hormone (AMH).