Testosterone is the primary hormone produced by the male body.  Testosterone levels normally decline decade by decade, but the decline is very small.  If the testosterone level is abnormally low for an adult male, he may experience symptoms of fatigue, change in body habitus (fat replacing muscle), low sex drive, erectile dysfunction, and others.  Determining why the testosterone level is abnormally low is very important, because the treatments to raise the testosterone back into the normal range may vary.