Androgen is a term used to describe any natural or synthetic hormone that stimulates or controls the development and maintenance of male characteristics. Testosterone is the most well-known androgen, and it is responsible for the development of male reproductive organs, secondary sex characteristics, and muscle mass. Testosterone is produced in the testes and is regulated by the hypothalamus and pituitary gland. It is also involved in the regulation of bone density, red blood cell production, and libido.