In Cushing's Disease (Hyperadrenocorticism), the adrenal glands produce too much of the hormone "cortisol", which causes negative effects to tissues and organs throughout the body.
Approximately 90% of the cases of dog and cat Cushing's Disease are caused by a tumor in the pituitary gland (pituitary microadenoma). These pituitary gland tumors cause an increase in the production of adrenocorticotrophic hormone (ACTH), which then signals the adrenal glands to produce an excessive amount of cortisol.
Cushing's Disease usually affects small breed and senior dogs.
(Elevated cortisol levels can also be caused by a tumor in one of the adrenal glands themselves. RightPet has a section for adrenal-caused hyperadrenocorticism -
Cushing's Syndrome Due to Adrenal Tumor.)