Testosterone is the predominant male sex hormone that is produced by the testes beginning in the fetus' eighth week.

In the fetal and embryonic stages of development, testosterone promotes the development of the penis and scrotum and the formation of the structures involved in sperm production.

In the pubescent years (age 9 to 14), testosterone helps in the growth of the testes, body hair, muscles and bones as well as sexual maturation and deepening of the voice.

In adulthood, testosterone may...