The LH (serum) test measures the level of luteinizing hormone (LH) in the blood, which plays a crucial role in regulating the menstrual cycle in women and testosterone production in men. Elevated or decreased LH levels can indicate various conditions, such as polycystic ovary syndrome (PCOS), menopause, or pituitary disorders. This test is often used to assess reproductive health, diagnose fertility issues, and monitor hormonal therapy.