Follicle-Stimulating Hormone (FSH)
Follicle-stimulating hormone is released by the anterior pituitary gland in response to signals from the hypothalamus. Its primary job is to stimulate the growth and maturation of ovarian follicles. Each month, FSH recruits a cohort of follicles, one of which typically becomes dominant and proceeds to ovulation.
Basal FSH, drawn on cycle day 2, 3, or 4, is a standard component of an ovarian reserve panel. An elevated basal FSH signals that the pituitary is working harder than normal to recruit follicles. This elevation reflects reduced ovarian sensitivity, which is often an early indicator of declining reserve. The threshold for "elevated" varies by laboratory and by age.
FSH is most useful in combination with other markers. A single elevated FSH in an otherwise normal cycle may be less meaningful than persistently elevated FSH across multiple cycles. Similarly, FSH must be read alongside AMH, AFC, and estradiol on the same draw date. Estradiol, if elevated on day 3, can suppress FSH artificially and mask the true reserve status.
In men, FSH plays a parallel role: it stimulates the Sertoli cells of the testes, which support sperm production. Elevated FSH in a man with low sperm counts often signals primary testicular failure, meaning the testes are not responding adequately despite the pituitary's signal. This finding changes the clinical approach significantly.
RRM clinicians use FSH trends over time rather than single-point snapshots. A rising FSH in a woman in her mid-30s is a call to investigate and act, not to reassign her to a lower-prognosis category and recommend moving to donor egg.
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.