FSH vs LH: what each hormone does, what the levels mean and whether the ratio matters

The hypothalamus releases GnRH in pulses; the pituitary answers with FSH and LH. Slow pulses favor FSH, fast pulses favor LH — which is one reason the balance shifts across the cycle and in PCOS. The two hormones act on different cells: in the ovary, FSH on the granulosa cells that grow the follicle and make estradiol, LH on the theca cells that make the androgen precursors and, at the surge, on the mature follicle to release the egg. In the testis, FSH acts on the Sertoli cells that nurse sperm and LH on the Leydig cells that make testosterone.

The same LH value can be normal on day 3 and an ovulation surge on day 14, so FSH and LH are only interpretable with the cycle day. For ovarian reserve the draw is day 2–5: an FSH above about 10 mIU/mL (with estradiol under 80 pg/mL, since a high estradiol suppresses FSH and hides the rise) suggests diminished reserve, above 25–40 on two occasions a month apart with no periods is menopause or, under 40, primary ovarian insufficiency. AMH now does the reserve job better because it does not vary with the cycle.

For ovulation the question is whether an LH surge happens; urine LH kits detect it a day or so before the egg is released, and a progesterone drawn seven days later confirms it. In hypothalamic amenorrhea — under-fueling, heavy training, stress — both FSH and LH are low or low-normal with a low estradiol; in menopause both are high, FSH higher than LH.

A low testosterone is only half a diagnosis; the LH and FSH say where the fault lies. High LH and FSH with a low testosterone mean the testes are failing and the pituitary is pushing — primary hypogonadism (Klinefelter syndrome, prior chemotherapy or radiation, mumps orchitis, trauma, ageing). Low or inappropriately normal LH and FSH with a low testosterone mean the signal is missing — secondary hypogonadism from obesity, opioids, anabolic steroid use, high prolactin, a pituitary tumor or iron overload. An isolated high FSH with a normal testosterone points to a sperm-production problem and belongs with a semen analysis.

Many women with PCOS have an LH above their FSH on day 2–5 (a ratio of 2 or 3 to 1), because fast GnRH pulses favor LH. But a third or more have a normal ratio, obesity blunts it, and a single LH drawn near a surge produces it in anyone, so the 2023 international PCOS guideline does not use the ratio. PCOS is diagnosed on irregular cycles, clinical or biochemical androgen excess, and ovarian morphology or AMH, with other causes excluded (TSH, prolactin, 17-hydroxyprogesterone). The ratio is a supporting clue, nothing more.