Luteinizing Hormone (LH)

The hormone that triggers ovulation each cycle.

LH is the hormone that sets ovulation in motion. It stays low for most of the cycle, then surges sharply about 24 to 36 hours before an egg is released. That surge is what an ovulation test strip detects — and why a positive result is one of the most useful fertility signals you can measure at home.

What LH does

Luteinizing hormone is made by the pituitary gland, working in partnership with FSH. While FSH matures the follicle through the first half of the cycle, LH delivers the final instruction: release the egg. After ovulation, LH also helps the empty follicle produce progesterone.

Why levels surge

When a follicle is mature, rising estradiol flips a switch in the brain, and the pituitary releases a burst of LH. The surge is brief — typically detectable in urine for about a day — which is why daily testing around midcycle matters for catching it.

What a result can tell you

A positive LH test means ovulation is likely within the next 24 to 36 hours — the most fertile window of the cycle. Whether you’re trying to conceive or simply want to understand your cycle’s rhythm, tracking the surge turns guesswork into a pattern you can actually see.

What it can’t tell you

An LH surge predicts ovulation; it doesn’t confirm the egg was actually released — that’s where a PdG test adds the missing half of the picture. Some conditions, like PCOS, can produce elevated baseline LH and confusing results. If your pattern doesn’t make sense across a few cycles, it’s worth reviewing with a healthcare provider.