Testosterone is a hormone for sex that plays important roles within the body. It is thought that testosterone regulates libido, the mass of bones, the distribution of fat and the strength of muscles. It also assists in the production of red blood cells and sperm. A tiny portion of the circulating testosterone is converted to estradiol, which is a type of estrogen. As we age, men often make less testosterone, which means they produce less estradiol well. So, changes commonly attributed to testosterone deficiency might be partly or entirely due to the decline of estradiol.