Addison's disease is a rare endocrine disorder that affects the adrenal glands, which are responsible for producing hormones that regulate metabolism, the immune system, and other important bodily processes. Symptoms of this disorder can include fatigue, low blood pressure, darkening of the skin, low blood sugar, and muscle weakness. Treatment for Addison's disease includes hormone replacement therapy and lifestyle modifications. If left untreated, Addison's disease can lead to life-threatening complications.