Addison's disease occurs when the adrenal glands (located near the kidneys) fail to produce enough hormones. The adrenal glands normally produce several hormones that control body function. For example:
Cortisol: a hormone responsible for stress responses
Aldosterone: a hormone responsible for balancing electrolytes (such as sodium and potassium)
It is most common in young female dogs, and you can manage it with hormone supplements.
Humans and cattle can also have this disease.