The most common manifestations of testosterone deficiency (hypogonadism) are a loss of sex drive and decreased bone mineral density.

Erectile dysfunction (ED) can also accompany hypogonadism, but ED can also be caused by a number of other factors, especially as men grow older. Mood changes, such as depression, and fatigue can be manifestations of severe testosterone deficiency.

Now, more and more studies are showing that men with diagnosed testosterone deficiency are more likely to suffer from a complex of conditions called the metabolic syndrome. And the metabolic syndrome - characterized by obesity; high LDL (so-called bad) cholesterol, triglycerides and blood pressure; and insulin resistance - is associated with a higher risk of developing type 2 diabetes and heart disease.

More than 5 million American men are estimated to have hypogonadism, a percentage that increases with age. However, only about 5 percent of all men with hypogonadism are thought to receive testosterone replacement therapy.

Men of all ages should consider asking their doctor for a test to measure blood testosterone levels if they become aware of a loss of sexual drive. Hypogonadism is diagnosed when blood testosterone levels are less than 200 nanograms (ng)/dL in a man with typical symptoms. Some men with testosterone deficiency have testosterone levels between 200 and 300 ng/dL. 

Testosterone replacement usually corrects these abnormalities and symptoms of hypogonadism. However, no clinical trials have proven that testosterone treatment decreases cardiovascular events or deaths. There is concern that testosterone replacement might increase the spread of hidden areas of prostate cancer. Thus far, studies have not shown a higher risk of prostate cancer with testosterone administration. Nonetheless, men should have a PSA before getting testosterone. 

Testosterone cannot be taken by mouth. Replacement therapy requires testosterone injections every two to three weeks or daily application of a testosterone patch or testosterone gel.

Although men with diagnosed low testosterone can benefit from testosterone replacement, older men with normal testosterone levels do not benefit from the testosterone supplements that are widely advertised on the Internet.