Testosterone is the chief androgen sex hormone in men, of which 95 percent is made in the testicles and the other 5 percent in the adrenal glands, located at the upper part of the kidneys. Testosterone is a hormone created by the testicles, and it is reliable for the correct development of the male sexual attributes. Testosterone is also important for maintaining muscle bulk, adequate levels of red blood cells, bone density, sense of well-being, and sexual and reproductive function. There is a steady reject as men get older.