Testosterone’s effects can be felt above the belt, too. A 2008 Archives of General Psychiatry study found that older men with the lowest levels of free testosterone were at a higher risk for depression than men with the highest levels. Researchers haven’t pinpointed the link, but other studies have shown that waning testosterone levels may lead to increased fat mass, irritability, and a lower sex drive.

Testosterone: Not Just for Libido

Testosterone is the most important metabolic hormone for men and many do not realize that it affects much more than just libido. Findings reported in the Wall Street Journal in December 2009 reveal that men with low testosterone tend to be tired and grumpy, have less energy, less strength, in addition to less interest in sex than they used to have. They also have a high risk of memory problems, obesity, high blood pressure, high cholesterol and are more likely to have cardiovascular disease.

You can not maintain or build muscle mass without having free, unbound testosterone in your body.