Testosterone is the principle male sex hormone and an anabolic steroid (anabolic steroids increase protein synthesis and build up muscles. They help to increase muscle mass, growth of body hair and secondary sexual characters of men). It is secreted in testicles. A small amount of this hormone is also secreted by adrenal glands.

In men testosterone plays an important role in complete development of male reproductive tissues like testes, prostate etc. Testosterone is largely responsible for activating the formation of male-body parts during development in the womb such as the penis, scrotum and prostate

It also promotes development of secondary sexual characters like beard, mustache, body hair, pubic hair, growth of penis and testicles, hoarseness of voice etc during puberty. Testosterone also activates an increase in the length of the bones, increasing overall height.

During puberty it influences penile erections. Erections become more frequent. Spontaneous ejaculation of semen or nocturnal emissions or wet dreams may occur during this time. Testosterone plays an important role in male fertility. Normal levels of testosterone help to increase libido, semen production, good erections and ejaculation.