Male Facts.. Testosterone is the primary male sex hormone and androgen in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. What does the testosterone do to the body? Testosterone is a sex hormone that plays important roles in the body. In men, it's thought to regulate sex drive (libido), bone mass, f