Testosterone is the most important sex hormone in the male body. It is needed for masculine growth and development during puberty, and the development of male characteristics such as body and facial hair, muscle growth, strength and a deep voice. Normal levels of testosterone also influence the production of sperm, promote sexual function and sex drive. The brain and the testicles work together to keep testosterone levels within a normal range. When levels of testosterone are below normal, the brain signals the testicles to make more. When testosterone levels are too high, the brain signals for the testicles to make less.