Testosterone hormone molecule. Computer model showing the structure of a molecule of the male sex hormone testosterone. Testosterone is the main human androgen, the class of steroid hormones responsible for the development of the male reproductive organs and secondary sexual characteristics. It is mainly produced in the testes. Testosterone may be used in hormone replacement therapy to treat delayed puberty in boys, some types of impotence, and breast cancer in women. Atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), and oxygen (red) - with the bonds between them as rods (silver).