Addison's disease occurs when the adrenal glands do not produce enough (or any) of the hormones, cortisol and aldosterone. These adrenal gland hormones are necessary for balancing water and energy in the body. Symptoms usually develop slowly over time, and may include fatigue, loss of appetite, abdominal pain, and dark patches of skin. Sometimes symptoms occur suddenly causing a condition called acute adrenal failure, also known as an acute adrenal crisis. Symptoms of an acute adrenal crisis include sudden weakness, pain, and fainting. The most common cause of Addison's disease is an abnormal response of the body's immune system. Injury to the adrenal gland can also cause Addison's disease. Diagnosis is based on the the symptoms, blood and urine tests that evaluate adrenal function, and imaging studies.