Cushing’s syndrome is one of the most commonly diagnosed canine endocrine disorders. Cushing’s syndrome is usually seen in older dogs and can be either spontaneous or iatrogenic.

The signs of Cushing’s syndrome in dogs are due to an excess of circulating glucocorticoids, particularly cortisol. Cortisol is a hormone produced in the cortex of the adrenal glands. The normal function of cortisol is to help the body respond to stress and change, and also to prepare the body for the fight or flight response. Cortisol mobilizes nutrients, modifies the body’s response to inflammation and stimulates the liver to raise the blood sugar.

There are two forms of spontaneous Cushing’s syndrome:

Pituitary-dependent hyperadrenocorticism (PDH)
This is the most common form of the disease (80% to 85% of spontaneous cases), and it often occurs as the result of a benign tumor of the pituitary gland. This tumor causes the pituitary gland to produce large amounts of ACTH, which stimulates the adrenal glands to produce excessive amounts of cortisol.

Adrenal-dependent hyperadrenocorticism (ADH)
This form of the disease is less common (15% to 20% of spontaneous cases), occurs when a tumor develops in one, or sometime both, adrenal glands. This tumor produces excessive levels of cortisol.

Take a look at Dr. Peterson's Blog - Insights into Veterinary Endocrinology

Dr. Mark E. Peterson is a world-renowned specialist in veterinary endocrinology, specifically diabetes, hyperthyroidism, hypothyroidism, Cushing's and Addison's disease. Dr. Peterson served as head of endocrinology and nuclear medicine at The Animal Medical Center in New York for over 30 years. He now sees patients at his private practice, the Animal Endocrine Clinic (New York City and Westchester County), the only veterinary hospital devoted entirely to animals with endocrine problems.

http://www.drmarkepeterson.com/insights-into-veterinary-endoc