Testosterone is a hormone produced by the body and contributes to the development of the sexual organs as well as muscle development and body hair. Normal levels of testosterone contribute to energy, sexual function, mood and libido (the sexual drive). However, in some men, this system fails due to a number of reasons. This condition is called hypogonadism and can result from a variety of causes, including side effects from certain medications and testicular injury. Whatever the cause, low testosterone levels can result in decreased sexual desire, diminished energy and depressed mood.