There are three main causes of Cushing’s disease in dogs, and each requires a different treatment approach. Understanding the difference helps you make sense of the tests your veterinarian recommends and the plan they suggest.
Pituitary-Dependent Hyperadrenocorticism
This is by far the most common cause, accounting for roughly 80–85% of all cases of Cushing’s disease in dogs. It involves an excess secretion of the hormone ACTH by the pituitary gland, usually because of a small, typically benign tumor there. ACTH is the signal that tells the adrenal glands to produce cortisol, so when there is too much ACTH, both adrenal glands are constantly stimulated to make far more cortisol than the body needs.
Most pituitary tumors are tiny (microadenomas) and do not spread. In a smaller number of dogs the tumor is larger (a macroadenoma) and can eventually press on nearby brain tissue, which is why your veterinarian may occasionally recommend advanced imaging.
Adrenal-Based Hyperadrenocorticism
In roughly 15–20% of cases, the problem lies in the adrenal gland itself. A tumor develops on one of the two adrenal glands and produces cortisol on its own, regardless of the ACTH signal coming from the pituitary. An abdominal ultrasound of a dog with adrenal-based Cushing’s often shows one enlarged adrenal gland while the other appears small or normal. Adrenal tumors may be benign or malignant, which is one reason accurate diagnosis is so important.
Iatrogenic Hyperadrenocorticism
The third cause is iatrogenic, meaning it is caused by medical treatment rather than by disease. If a dog has allergies, immune conditions or another chronic problem that requires long-term glucocorticoid (steroid) medication, that steadily added cortisol-like medication can flood the body and produce the same signs as a natural over-production. In effect, the excess corticosteroids mimic what an adrenal or pituitary tumor would do.
The encouraging part is that iatrogenic Cushing’s is usually the most straightforward to resolve: under veterinary supervision, the steroid is gradually and carefully tapered (never stopped abruptly), which allows the adrenal glands to recover their normal function over time.
Why the Cause Changes Everything
Because the three causes are so different, they are not treated the same way. A pituitary-dependent case is usually managed with daily medication; an adrenal tumor may be a candidate for surgery; and an iatrogenic case is addressed by adjusting the medication that triggered it. This is exactly why the diagnostic testing your veterinarian performs is worth doing carefully — it is not just confirming the disease, it is identifying which kind your dog has. Board-certified specialists in this area, such as those recognized by the American College of Veterinary Internal Medicine, focus specifically on complex hormonal conditions like this one.
Next, learn to recognize the symptoms of Cushing’s disease, or read how veterinarians confirm a diagnosis.