Luteinizing hormone (LH) is produced by the pituitary gland. Along with other hormones (follicle-stimulating hormone and estrogen), LH helps regulate the menstrual cycle in women and causes ovulation to occur. The amount of LH released into the body varies with the phase of the menstrual cycle. It increases rapidly just before ovulation occurs, about midway through the cycle (day 14 of a 28-day cycle). This is called an LH surge. Luteinizing hormone and follicle-stimulating hormone levels rise and fall together during the monthly menstrual cycle. In men, LH stimulates testosterone production and is necessary for sperm to mature. LH is usually measured in both a man and a woman to determine why the couple cannot achieve pregnancy (infertility).