LH (Luteinizing Hormone) — Definition, LH Surge, Ovulation & Fertility Role

LH (luteinising hormone) is produced by the pituitary gland and plays a central role in triggering ovulation. Each month, as the dominant follicle in the ovary grows and produces increasing levels of oestrogen, the pituitary responds with a sudden, sharp spike in LH — called the "LH surge." This surge triggers the final maturation of the egg and causes the follicle to rupture and release the egg approximately 36–40 hours later. Without the LH surge, ovulation does not occur.

The LH surge is a sharp rise in LH (typically to 20–100 IU/L, from a baseline of 2–8 IU/L) that occurs roughly 36–40 hours before ovulation. It usually occurs on Day 13–15 of a 28-day cycle, but can vary significantly. Detection methods: (1) Urine ovulation predictor kits (OPKs) — test from Day 10; positive OPK = surge detected; best tested mid-morning (not first morning urine); intercourse on the day of the positive OPK and the following day; (2) Digital OPKs (Clearblue) — show "flashing" (high oestrogen) then "static smiley" (LH surge) — identify a 4-day fertile window; (3) Serum LH — blood test; used in monitored IUI and natural cycle FET for precise timing.

A Day 3 LH above 10 IU/L, particularly if the LH:FSH ratio is >2:1 or 3:1, is a classic sign of PCOS (polycystic ovary syndrome). In PCOS, the pituitary secretes chronically elevated LH, which disrupts follicle development and prevents normal ovulation. Elevated basal LH can also occasionally be seen in: poor ovarian reserve (where the pituitary is overworking); premature LH rise (where LH starts rising before the dominant follicle is fully mature — common in stimulated IVF cycles, hence the need for GnRH agonist or antagonist to prevent it). If your Day 3 LH is elevated, your doctor will also check FSH, AMH, testosterone, and do a pelvic ultrasound to assess for PCOS.

LH plays multiple roles in IVF: (1) Suppression during stimulation: GnRH antagonist injections (Cetrotide, Orgalutran) suppress the pituitary from releasing LH prematurely during stimulation — preventing premature ovulation before egg retrieval; (2) Triggering ovulation: when follicles are mature (≥17–18mm), ovulation is triggered using either hCG injection (which acts like LH at the LH receptor) or a GnRH agonist injection (which causes the pituitary to release its own LH/FSH surge); egg retrieval is performed 36 hours later; (3) LH supplementation: some patients (especially poor responders and women >35) benefit from adding LH activity (as rLH or hMG) alongside FSH during stimulation to improve follicle quality.

Persistently elevated LH on OPKs (always positive, or never clearly negative) is commonly seen in PCOS. In PCOS, baseline LH is chronically elevated — so the "background" LH level may already be near or above the OPK detection threshold, making it difficult to identify the true LH surge. This makes OPK-based timing unreliable in PCOS. Better options: (1) Serum LH blood tests (measured daily from Day 10 in clinic) — gives a precise numeric value to identify the true surge above the elevated baseline; (2) Monitored follicle tracking with TVS — identifies the dominant follicle; a triggered cycle (hCG injection when follicle ≥17mm) removes the guesswork entirely and is preferred for PCOS-related IUI cycles.