Testosterone. Computer artwork of a molecule of the male sex hormone testosterone (chemical formula C19.H28.O2). Atoms are depicted as spheres and are colour-coded: carbon (yellow), hydrogen (blue) and oxygen (red). 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.