Testosterone, the primary male hormone, is produced in the testes, and governs many functions throughout your body. It’s necessary for sex drive in both males and females, though women have much smaller amounts. In men, testosterone is responsible for normal development of sex organs, the production of sperm, and male characteristics like facial hair and a deep voice. Testosterone also affects bone and muscle growth, the prostate, and your central nervous system. Levels tend to peak early in the morning, and fall to their lowest point in late evening. As you age, testosterone production gradually declines. By age 60, some men’s levels become so low as to cause fatigue, lack of sex drive, and the inability to get or maintain an erection. Irritability, decreased muscle mass, reduction in facial hair, and increased breast size are other possible symptoms. Some younger men may also experience an abnormal drop in testosterone, a condition known as hypogonadism . In either case, hormone replacement therapy may be needed. The most recent method of supplementing testosterone levels is with a one-dose patch that’s worn on the arm, thigh, shoulder, or back. For more information on testosterone, consult a health care professional.