Synthesis of Testosterone Testosterone is the primary sex hormone and anabolic steroid 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. Testosterone is a steroid from the androstane class containing a ketone and a hydroxyl group at positions three and seventeen respectively. In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males and from ovaries of females in lesser extent.