LH Female
Luteinizing hormone (LH) surges 24 to 36 hours before ovulation and triggers the dominant follicle to release the egg. Day-3 baseline LH together with FSH gives the LH:FSH ratio, which is the clinical signature of polycystic ovary syndrome (PCOS): typically above 2:1 in PCOS and roughly 1:1 in normal cycles. Mid-cycle LH is what at-home ovulation predictor kits detect and what fertility-tracking apps key off.
Why this biomarker matters
In cycling women, the day-3 LH:FSH ratio reflects the balance of pituitary outputs that orchestrates each menstrual cycle. An elevated ratio (LH disproportionately high relative to FSH) is one of the laboratory features of PCOS, alongside hyperandrogenism, oligo-ovulation, and polycystic ovarian morphology on ultrasound. It tracks roughly with insulin resistance and the broader metabolic features of PCOS, although it is not sensitive enough to rule the diagnosis in or out alone. The mid-cycle LH surge is the trigger for ovulation. Detecting it with serum or urine LH allows precise timing of fertility windows, which is why LH strips dominate the at-home fertility-tracking market. Sustained elevation of LH with low estradiol in menopausal-range patterns reflects loss of ovarian feedback and is one of the laboratory features of menopause. Persistent low LH with low estradiol, hypogonadotropic hypogonadism, flags hypothalamic suppression from underfueling, excessive training, prolactinoma, or central pathology, all of which warrant evaluation.
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