Luteinizing hormone (LH) is produced by the pituitary gland. In women, LH helps regulate the menstrual cycle and egg production (ovulation); in men, LH stimulates the production of testosterone, which plays a role in sperm production.
Women's LH levels normally vary with the phase of the menstrual cycle, rapidly increasing just before ovulation occurs. This "LH surge" is a dependable sign that a woman is in a fertile time of her cycle. Men's LH levels normally remain constant.
Abnormally high or low LH levels can be a sign of an inability to produce eggs or sperm.