Testosterone is a sex hormone in the body that assists in the development of a mans sexual organs, including sperm and secondary sexual traits. It is important for sexual and reproductive development. In men, testosterone is produced mainly in the testicles, with a small amount made in the adrenal glands. Testosterone is involved in the development of male sex organs prior to birth. Also, the development of secondary sex characteristics at puberty, such as voice deepening, increased penis and testicle size, and increased facial and body hair. According to the Mayo Clinic, testosterone also plays a role in sex drive, sperm production, fat distribution, red cell production, and maintenance of muscle mass and strength.