Low testosterone, or hypogonadism, can be caused by many factors, all of which play out against the normal steady decline in testosterone levels with age.
Tumors on the pituitary gland, which controls testosterone production in the testicles; problems with the testicles themselves; injury, infections; and being overweight can all cause testosterone levels to drop below normal.
Excess body fat does this because testosterone is normally broken down in the body’s fat cells. If you have a lot of fat, your body breaks down testosterone extra-quickly, leading to a deficiency. Abdominal or “belly” fat has a greater capacity to convert testosterone to estrogen than other types of fat.
Another risk factor for hypogonadism that has only recently come to light is diabetes. A strong relationship has been discovered between impaired glucose tolerance, which is a cardinal feature of diabetes, and low testosterone levels.
It appears that the high blood sugar levels and/or low insulin levels characteristic of diabetes harm the cells in the testicles that are responsible for making testosterone.