Testosterone is a hormone made by the body and is responsible for the normal growth and development of the male sex organs and for maintenance of other sexual characteristics. It's what puts hair on a man's chest. It's the force behind his sex drive. In adulthood, it keeps a man's muscles and bones strong and maintains his interest in sex. In short, it's what makes a man a man (at least physically). After age 30, most men begin to experience a gradual decline in testosterone. A decrease in sex drive sometimes accompanies the drop in testosterone, leading many men to mistakenly believe that their loss of interest in sex is simply due to getting older. Many of the symptoms of low testosterone are mirrored with other medical problems; that is, other medical problems may arise as a result of low testosterone; including, but not limited to, diabetes, depression, high blood pressure, and coronary artery disease – many of these stemming from obesity.
There is a progressive decline in testosterone production in men with age. These changes can be dramatic, such that 50% of men over 60 years old have low levels of testosterone. Although the rate of decline varies widely, a general rule of thumb is that testosterone decreases about 1% yearly after age 50.
Testosterone affects the function of many organs in the body. In the brain, it influences sex drive, male aggression, mood and thinking. Testosterone can improve verbal memory and visual-spatial skills. It has also been shown to decrease fatigue and depression in men with low levels. It is responsible for muscle strength and growth, and stimulates stem cells and blood cells in bones and kidneys. Penile growth, erections, sperm production, and pro-static growth and function all depend on testosterone. It also causes body hair growth, balding and drives beard growth, thus, testosterone makes us who we are and influences how we look.