Testosterone is a steroid hormone and is the principal male sex hormone and an anabolic steroid. It is secreted mainly by the testicles of males and small amounts are also secreted by the adrenal glands. Testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate and is essential for health and well-being, and for the prevention of osteoporosis. It also promotes the secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. After age 30, most men begin to experience a gradual decline in testosterone and a decrease in sex drive sometimes accompanies the drop.