Testosterone is a hormone that is produced by the human body. It is mainly produced in the testicles in men. It stimulates sperm production and a man’s sex drive and also helps build muscle and bone mass.Testosterone production typically decreases as men age. Men can experience a range of symptoms if it decreases more than it should. Low T is diagnosed when levels fall below a normal range (300- 1000 ng/dL, according to the U.S. Food and Drug Administration). A blood test (called serum testosterone level) is used to determine your level of circulating testosterone. A range of symptoms can occur if testosterone production drastically drops. Signs of low T (also called hypogonadism) are often subtle and can be mistaken for a natural part of aging.