Follicle Stimulating Hormone is produced by the anterior pituitary. It is responsible for testicular spermatogenesis and ovarian folliculogenesis, acting on its receptor on the sertoli cell, or granulosa cell, respectively. FSH is the most common endocrine ovarian reserve test; however it has significant limitations, and in most circumstances should be used only as a prognostic aid.

FSH secretion is under the control of gonadal sex steroids (estrogen in females and testosterone in males) as well as inhibin B. In the classic endocrine negative feedback loop of the hypothalamic-pituitary-ovarian axis, as estrogen production increases throughout the early follicular phase, FSH decreases. Exhaustion of the ovarian reserve at menopause results in significant elevations of FSH. Premature menopause, for example, can be diagnosed using elevated FSH (> 40 IU/L), repeated 4 weeks apart, in a woman under 40 years old.

FSH must be measured in the early follicular phase (cycle day 2 – 4). Concurrent measurement of estradiol (< 200 pmol/L) is recommended to confirm that the FSH is not being suppressed, and therefore falsely reassuring. When stratifying by female age, it appears IVF live birth rates are maximal when basal FSH level is < 7 IU/L, and at all ages, live birth rates are <2% when basal FSH levels are > 18 IU/L. Given the nature of the FSH test and its limited clinical utility in detection of spontaneous or treatment-induced fecundity, an FSH level should never be seen as reassuring, at any age. Research shows that rising FSH levels (above 7 IU/L) can demonstrate progressive cause for concern with each incremental unit elevation.