The luteinizing hormone (LH) test is a blood or urine test that measures the level of LH in the body. LH is a hormone produced by the pituitary gland that plays a key role in regulating the reproductive system. In women, LH is responsible for triggering ovulation and the release of an egg from the ovary. In men, LH stimulates the production of testosterone by the testes. The LH test is commonly used in fertility treatments to help determine the optimal time for conception. LH levels typically surge just before ovulation, which is a sign that a woman is most fertile. In addition to fertility testing, the LH test can also be used to diagnose conditions such as polycystic ovary syndrome (PCOS) or pituitary disorders that can affect reproductive function.