In a guy who has reached sexual maturity, the 2 testicles, or testes, produce and store millions of tiny sperm cells. The testicles are oval-shaped and grow to be about 2 inches (5 centimeters) in length and 1 inch (3 centimeters) in diameter. The testicles are also part of the endocrine system because they produce hormones, including testosterone. It is the principal male sex hormone and an anabolic steroid. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis. On average, the adult male body produces about twenty to thirty times the amount of testosterone that an adult female's body does.

Low testosterone, sometimes called hypogonadism or andropause, has been described as a symptom complex in the presence of low levels of testosterone that can include a decline in libido and erectile function, as well as irritability and loss of the ability to concentrate. As males age, there is both decreased production of testosterone and decreased testosterone clearance beginning at age 40.

Not all men decline below the normal limits for serum testosterone, but may have complaints that are associated with low plasma testosterone levels. However, many of the complaints that accompany the aging male have shown a weak correlation with plasma testosterone levels.

There are some very clear causes of hypogonadism. These include primary causes such as Klinefelter’s disease, chemotherapy, radiation treatment, loss of a testicle due to trauma or surgery, or failure of one or both testicles to descend into the scrotum. There are also secondary causes of low testosterone such as pituitary lesions, hemachromatosis, Kallman’s syndrome and psychological stress. Sickle cell disease and cirrhosis can also result in lowered testosterone levels, and aging can contribute as well.

Male impotence occurs more commonly in men with diabetes than others but it is still probably a minority of men with diabetes who experience it. In order to have normal erectile function, it is necessary to have adequate male hormone (testosterone)production, and adequate function of both nerves controlling the penis and blood vessels supplying the penis. Problems with those nerves and blood vessels both occur more commonly in men with long-standing diabetes. It is of interest that one of the other common health problems which also can cause problems with impotence is excessive alcohol use. Infertility, erectile dysfunction, decreased sex drive, fatigue, muscle loss or weakness, enlarged male breasts, decreased beard and body hair growth, and osteoporosis are possible complications of hypogonadism in adults.

Men with testosterone levels in the low-normal range are much more likely to have diabetes than those with higher levels, whether or not they are obese, researchers report in the journal Diabetes Care.

"Low testosterone levels are common among men with diabetes and there is growing evidence that low testosterone may be a risk factor for developing diabetes," Dr. Elizabeth Selvin from Johns Hopkins Bloomberg School of Public Health in Baltimore commented to Reuters Health.

She and her colleagues analyzed data from 1,413 men 20 years or older who participated in the Third National Health and Nutrition Examination Survey. Diabetes had been diagnosed in 101 men.

After the influence of age, race and obesity was factored in, men with the lowest levels of testosterone were four times more likely to have diabetes than men with the highest levels. Similar results were found when only the bioavailable testosterone was measured.

The association persisted even after men with low total or free testosterone levels were excluded, suggesting, the researchers say, that the association is not entirely driven by failure to produce enough testosterone.

"Obesity affects testosterone levels; testosterone levels drop in men who are overweight or obese," Selvin pointed out. Yet, "even after accounting for the effect of obesity, low testosterone levels still appear to be an important risk factor for diabetes," she said.

These data, the researchers write, support the hypothesis that male hormones directly influence sugar metabolism and the development of insulin resistance, which is seen just before diabetes occurs, independent of the effects of obesity.

Selvin added that doctors should keep in mind that "low testosterone is a common finding among men with diabetes and may lead to other clinical problems.

Hypogonadism can be inherited. If any of these risk factors are in your family health history, inform your doctor. Be aware of and watch for signs and symptoms of hypogonadism.

Another study by the University of Sydney researchers collected information (tracked 609 men (average age 72.6) between 1989 and late 2005) about the men's bone mineral density, lifestyle habits, and blood levels of testosterone and estradiol (an estrogen) and found that fracture risk associated particularly with hip and non-vertebral fractures was much higher among men with low testosterone levels. "Measurement of [blood] testosterone provides additional clinical information for the assessment of fracture risk in elderly men;" they concluded (date: Jan. 2008).