Follicle-stimulating hormone (FSH) is a glycoprotein hormone secreted by the pituitary gland. Acting synergistically with luteinizing hormone, which like FSH is produced in response to gonadotropinreleasing hormone (GnRH) pulses from the hypothalamus, it regulates fertility in both men and women. In women, ovarian hormones suppress GnRH in a negative feedback loop. As estrogen and progesterone levels fall during menstruation, FSH levels rise to a peak on day 3 of the menstrual cycle, stimulating recruitment and growth of immature follicles in preparation for ovulation during the follicular phase. Levels decline again in response to inhibin B produced by the follicles. If levels remain abnormally high in the luteal phase, this can be a sign of ovarian insufficiency, while higher than normal FSH levels in the follicular phase may indicate premature ovarian failure. As a woman enters menopause, declining ovarian hormone production reduces the negative feedback on GnRH production and FSH rises to very high levels. FSH testing is therefore commonly used to assess menopausal status. Low levels are seen in polycystic ovarian syndrome. In men, FSH is essential for spermatogenesis; low levels can lead to low sperm counts resulting in infertility, while high levels can indicate primary testicular failure. Children with precocious puberty have higher levels than normal for children. FSH levels are also affected by disorders of the hypothalamus or pituitary. The reference range for blood spot FSH in premenopausal women (luteal phase) is 0.6—8.0 U/L, in premenopausal women (follicular phase) 2.4—9.3 U/L, in postmenopausal women 31—134 U/L, and in men 1.0—10.5 U/L.