Though the number one average cause of low testosterone is simply age, this is by no means the only cause. Many other causes contribute to an overall decreased level of testosterone aside from the loss that occurs with age.
Some individuals are born with different illnesses that result in testosterone deficiency (TD). This includes Klinefelter syndrome, Noonan syndrome, and ambiguous genitalia. Though the last item is not a syndrome like the other two, ambiguous genitalia still contributes to TD because it denotes altered genital development.
Some illness and circumstances do not result in TD – they result in Low-T. Low-T refers to a lowered level of testosterone that might result in some side effects, though for the most part it just acts as a risk factor for TD. In other words, Low-T implies that the individual should take steps to either fix their hormone levels or monitor closely.
Factors that contribute to Low-T include: physical damage to the genitalia, testicle removal, chemotherapy/radiation, pituitary gland issues, autoimmune diseases, HIV/AIDS, and infection.
In women, low testosterone levels are generally the result of some form of deficiency in development resulting in improper pituitary gland or adrenal gland functioning, or issues with the ovaries.
Additional risk factors associated with Low-T include obesity, metabolic syndrome (high blood pressure, high cholesterol, belly fat, and high blood sugar), and the use of antidepressants or narcotic medication. It seems that the most prevalent risk factor for altered testosterone levels, when unrelated to age or disease, is health. Extreme instances of obesity and/or metabolic syndrome contribute to Low-T even in cases where the individual does not possess any other risk factors.