Luteinizing hormone (LH) is a gonadotropic hormone secreted by the anterior pituitary gland. In males, it stimulates the interstitial cells of the testes to produce testosterone. Testosterone is responsible for the development of male reproductive organs and secondary sex characteristics such as muscle mass, bone density, and body hair. LH production is regulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus. In females, LH plays a critical role in ovulation, the process by which a mature egg is released from the ovary. LH levels surge just before ovulation, triggering the release of the egg. In males, LH secretion is relatively constant, with minor fluctuations, and stimulates the production of testosterone throughout life.