Testosterone. Computer artwork of a molecule of the male sex hormone testosterone (chemical formula: C19.H28.O2). Also known as 17beta-hydroxy -4-androsten-3-one. Atoms are depicted as spheres and are colour-coded: carbon (blue), hydrogen (green), oxygen (pink). Testosterone is the main human androgen, the class of steroid hormones responsible for causing the development of male reproductive organs and secondary sex character- istics. 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.