Hormones are chemical messengers that regulate functions in the body. Testosterone is a type of androgen hormone and is responsible for the development of male sexual characteristics. Testosterones also regulate sperm production, sex drive, bone mass, fat distribution, muscle size, and red blood cell production. Testosterone hormones are produced primarily by the testicles while the brain and pituitary gland regulate the amount of testosterone levels being produced. Once produced, the testosterone hormone moves through the blood to carry out its various important functions.