Addison's disease is the most common form of adrenal insufficiency, accounting for 70-80% of all cases. The prevalence is relatively low, only 40-50 cases per million, and it tends to affect young individuals in their third or fourth decade. It can present as an isolated disorder or in combination with other autoimmune diseases. It is most commonly seen as part of a polyglandular syndrome, which accounts for up to 40% of the cases. Idiopathic Addison's disease is usually slowly progressive, with the development of clinical manifestations such as salt wasting, hypotension, anorexia, malaise, and hyperpigmentation occurring so gradually that they can easily be undetected. Serum levels of adrenocorticotropic hormone (ACTH) are often elevated long before clinical disease develops. The finding of small, noncalcified adrenal glands on x-ray or computed tomography of the abdomen helps to differentiate this disorder from adrenal insufficiency secondary to carcinoma (primary or metastatic) and tuberculosis. The laboratory diagnosis rests on the lack of a cortisol (and possibly aldosterone) response to ACTH administration.