Men and women alike produce the sex hormone testosterone. Men, however, have a much higher percentage of testosterone, which is mostly produced in the testicles. The hormone helps develop and maintain a man's sexual features and his sexual function; plus, it plays a role in muscle mass, levels of red blood cells, fat distribution and bone density.

Testosterone levels peak during adolescence and decline in later life but generally stay between 300 and 1,000 nanograms per deciliter. When the level falls below 300 ng/dL, the condition is considered low testosterone (low-T), or male hypogonadism. Low-T is common, affecting approximately 40 percent of men over the age of 45.

What causes low testosterone?

There are many potential causes of