Testosterone is a sex hormone that is responsible for aiding in the development of the male sexual characteristics. Females also have minute concentrations of the hormone, but it is mostly found in males and is considered to be the principal male hormone.
In males, testosterone is produced by the Leydig cells in the testes. It is then circulated throughout the entire body to help produce sperm and regulate the functions of the male reproductive system. Testosterone is also thought to play a part in the male sex drive, bone mass, fat distribution, muscle size and strength, and red-blood-cell production in males.
As men get older, their testosterone levels begin to drop. Researchers have found that men's testosterone drops approximately 1.6 percent each year after the age of 40. Low testosterone can produce a variety of symptoms, including a lower sex drive, diminished erectile quality, changes in mood or behavior, reduced cognitive abilities, fatigue, depression, decrease in muscle mass, less body hair, decreased bone mass and an increase in abdominal fat. In addition to these symptoms, low testosterone has also been linked to an increased risk of developing metabolic disease or cardiovascular disease. For these reasons, some men choose to take testosterone supplements as they grow older.