Testosterone is responsible for many things such as hair growth, sex desire, and erection. Testosterone is produced by the testicles from puberty onward. It is the hormone responsible for all male sexual characteristics and it affects energy levels to a great degree as well.
By the age of 30, testosterone levels start decreasing. By 40, the normal level of testosterone starts decreasing by about 1 to 2% annually. By 50, the symptoms of low testosterone levels start appearing including libido, erectile dysfunction, weight gain, muscle loss, sleep problems, mood swings, low bone mass, hair loss, and fatigue of course.