Cushing's disease usually cannot be cured, but dogs often respond well to treatment and can live a fairly normal life for years after diagnosis. However, the prognosis does depend on the cause of the syndrome. Dogs can develop Cushing's syndrome in three ways: it can be caused by a tumor in the adrenal gland, a tumor in the pituitary gland or by the long-term use of steroids to control other diseases.
Pituitary gland tumors are the most common cause of the disease in dogs, and the prognosis depends on whether the tumor is benign or malignant. Dogs with benign tumors generally have a reasonably good prognosis, although it can be difficult to find the right combination of medications to manage Cushing's syndrome.
Adrenal gland tumors are less common and require surgical intervention. Dogs with benign tumors have a very good prognosis if they survive the surgery, while those with malignant tumors generally have a poor prognosis unless the surgeon is able to remove all of the cancerous cells.
Iatrogenic Cushing's syndrome, which is the kind caused by steroids, can have a variable prognosis depending on why the steroids were given in the first place. Treatment requires the dog to be weaned off of the drugs, which usually causes the other disease to flare up again. If that illness is minor or has other treatments, the prognosis is generally good.