Testosterone is the main male sex hormone that is produced in the testes. One of its functions is to stimulate (“wake up”) the development of the male internal genital structures in the fetus. Testosterone is converted in the external genital tissues to a stronger hormone, dihydrotestosterone, to cause male development of the external genitals. It is also produced during a brief period in early infancy, then again at puberty, when it stimulates enlargement of the penis, deepening of the voice and other typical features of male secondary sex characteristics. During adult life it is responsible for maintenance of male-type body structure and is involved in sex drive (libido) and sexual function.