Testosterone is a male hormone that helps to maintain bone density, fat distribution, muscle strength and mass, red blood cell production, sex drive and sperm production.  As men age, levels of testosterone naturally decline by about 1% per year after age 30. Extremely low levels of testosterone may be caused by a disease (hypogonadism) in which the body isn’t able to produce enough of the hormone because of a problem with the testicles or the pituitary gland which promotes testosterone production.