Testosterone is the main androgen (male sex hormone). It stimulates the development of male characteristics, such as facial hair and voice deepening. Testosterone is essential for sperm production. In men, it helps develop muscle mass, bone density, red blood cells, sex organs, sexual and reproductive function, and a sense of well-being. The growth of primary sexual characteristics, such as a penis and testicles, is mediated by testosterone, both in the womb and during puberty.