Erectile dysfunction can be a symptom of an underlying problem in the physiological chain.  With your doctor, you have to figure out where the problem is occurring. 

The process of erection is complex and involves the vascular system, the nervous system, the endocrine system and an individual's emotions. 

When a man is sexually aroused the brain sends messages along the nerves to the vascular system. The messages tell the vascular system to increase the amount of blood flowing into the penis. Inside the penis are two chambers filled with spongy erectile tissue. This tissue fills with blood causing the penis to expand. At the same time, fibrous elastic sheathes around the erectile tissue tighten, so the blood can't leave the penis and the erection continues. After ejaculation or when sexual stimulation ends, the sheathes loosen, the blood leaves the penis, and the penis returns to its non-erect state. A problem with any one of these functions can result in ED. 

So, ED can be a symptom of many issues.  (See Key Point #1 for more information.)  That being the case, your doctor will want to do a thorough exam.

Elements of a medical exam for ED
A complete medical exam for ED may include some or all of the following:

• Medical history, including your general physical and emotional state, your lifestyle and questions about any prescribed or illicit drugs you're currently taking that might cause ED
  
• Sexual history
• How often do you have relations with your partner?
• How important is sex to you and your partner?
• Do you have difficulty obtaining and/or maintaining an erection?
• How long have you been unable to obtain and/or sustain an erection?
  
• Physical exam
• Are testicles normal in size?
• Does the prostate appear to be enlarged?
• Are hair patterns normal? (to evaluate potential problems with testosterone levels)
• Is the thyroid enlarged?
• Is blood pressure elevated?
• Does the body mass index suggest the risk of diabetes?
• Is there sensation in all extremities?
  
• Laboratory tests, such as routine blood and urine tests that can check your level of male hormones, cholesterol, blood sugar, and liver, kidney and thyroid function.
  
• Cardiac work-up including and EKG and echocardiogram. ED can be the first indicator of coronary artery and valve disease.
  
• Erectile function tests to evaluate blood flow into and out of the penis and measure the blood pressure in it (normally and during erection) and evaluate the sensitivity and nerve function of the penis. Among the most common are:
• Ultrasound to evaluate blood flow, venous leak, signs of arteriosclerosis and scarring or calcification of erectile tissue.
• Penile nerve function to determine if there is sufficient nerve sensation in the penis. 
• Nocturnal penile tumescence (NPT) tests to measure the number and strength of the erections you have during sleep.
• Vasoactive injection to measure pressure in penile blood vessels.

The importance of a proper diagnosis
Some patients are unwilling to undergo a long evaluation and testing process to obtain a better understanding of their sexual problem.  However, proper diagnosis is the key to beneficial treatment.  That's because:

• Many of ED's causes are physical and can be successfully treated; however, if they're not treated they may lead to other problems.
• Self-treatment with over-the-counter or over-the-internet remedies is usually ineffective and can even be dangerous.
• New, highly touted prescription medications for ED may or may not work, depending on the underlying cause of the problem.  If a patient's health is optimized, the drugs may not be needed – or, if needed, they may be more effective.