Testosterone replacement therapy is used to help regulate hormone levels in men with low testosterone, also known as Low-T. The testosterone hormone plays a role in sperm production, sex drive, fat distribution, maintenance of muscle strength, and red cell production. The testes produce testosterone, but the hypothalamus and pituitary gland in the brain control the amount of testosterone to produce. Testosterone levels gradually decrease with age, although some men will experience low levels of testosterone related to a problem with the pituitary gland or the testicles that prevent the body from producing normal amounts of testosterone, a disease known as hypogonadism.