On the other hand, a few men with normal or low normal testosterone levels will report improved erections with testosterone treatment. Most men with low serum testosterone levels who have erectile dysfunction report very little effect of testosterone therapy on their erections. At best, testosterone may enhance an erection, but it has been clearly shown that it is not essential for an erection. The primary male hormone testosterone is produced in the testicles and is responsible for male sex drive, libido, as well as growth of facial hair and genital hair. In patients with chronic diabetes, we can expect from 30 to 70 percent of diabetic males to be affected with erectile dysfunction, due to the effect of the disease on both blood vessels and nerves.

This disorder, as we have seen, does not cause impotence directly through any resulting hormonal deficiency, but from its effects on the vascular and neurological, as well as psychological components. The most common endocrinologic cause of impotence is diabetes mellitus. Another common source of neuro-genic-related impotence is found in patients who have had radical prostate cancer surgery, which will be discussed more fully in the section on prior surgery. This usually takes a very significant amount of alcohol intake over a long period of time - years, not a lost weekend. Chronic long-standing alcoholism can result in the loss of an adequate number of neurotransmitters in the penile nerves. Patients with high spinal cord injuries may still have a reflex erection.

In looking at spinal cord injury patients, the degree of erectile dysfunction depends on the degree of the injury as well as the location. Since Parkinson's patients also lose noradrenaline cells in the brain, in addition to the well-documented dopamine cells, this leads to a failure of the autonomic nervous system to help constrict the veins draining the erectile tissue, thus preventing a full erection. The main cause of erectile dysfunction in Parkinson's patients is autonomic nervous system insufficiency. Thus, any middle-aged male who has erectile dysfunction and has no other medical problems or risk factors for impotence, who has "slowed down" recently, should be worked up for Parkinson's disease. More importantly, erectile dysfunction may be the first symptom of Parkinson's disease, a fact often appreciated only in retrospect. However, in those 15 percent of men under age fifty who have Parkinson's (some 30,000 men), the erectile dysfunction is usually caused by the Parkinson's.