Testosterone is a hormone produced by the testicles and is responsible for the proper development of male sexual characteristics. Testosterone is also important for maintaining muscle bulk, adequate levels of red blood cells, bone growth, a sense of well-being, and sexual function.

Inadequate production of testosterone is not a common cause of erectile dysfunction; however, when ED does occur due to decreased testosterone production, testosterone replacement therapy may improve the problem.

As a man ages, the amount of testosterone in his body naturally gradually declines. This decline starts after age 30 and continues throughout life. Some causes of low testosterone levels are due to:

Without adequate testosterone, a man may lose his sex drive, experience erectile dysfunction, feel depressed, have a decreased sense of well-being, and have difficulty concentrating.

Low testosterone can cause the following physical changes:

The only accurate way to detect the condition is to have your doctor measure the amount of testosterone in your blood. Because testosterone levels fluctuate throughout the day, several measurements will need to be taken to detect a deficiency. Doctors prefer, if possible, to test levels early in the morning, when testosterone levels are highest.

Note: Testosterone should only be used by men who have clinical signs and symptoms AND medically documented low testosterone levels.

Read more:
Testosterone Replacement Therapy - WebMD