Cushing’s disease in dogs is caused by the body producing too much cortisol, a stress hormone made by the adrenal glands. In about 80 to 85% of cases, the root cause is a small tumor on the pituitary gland at the base of the brain. The remaining 15 to 20% of cases come from a tumor on one or both adrenal glands. A third, often overlooked cause is long-term use of steroid medications, which can trigger the same symptoms without any tumor at all.
Pituitary Tumors: The Most Common Cause
The pituitary gland acts as the body’s hormonal control center. One of its jobs is releasing a signaling hormone called ACTH, which tells the adrenal glands how much cortisol to produce. When a tumor develops on the pituitary gland, it sends out too much of this signal. The adrenal glands respond by overproducing cortisol around the clock. Over time, both adrenal glands physically enlarge from the constant stimulation.
This form, called pituitary-dependent hyperadrenocorticism, accounts for roughly 90% of naturally occurring cases. The tumors involved are usually very small (microadenomas), though in some dogs they grow large enough to press on surrounding brain tissue and cause neurological symptoms like disorientation or circling. Research from veterinary neurology has identified that dogs with pituitary Cushing’s show a depletion of dopamine-producing nerve activity in key brain regions, which may be part of what allows the tumor cells to overproduce ACTH in the first place.
Adrenal Gland Tumors
In 15 to 20% of dogs with Cushing’s, the problem isn’t in the brain at all. Instead, a tumor grows directly on one of the two adrenal glands (small organs that sit just in front of the kidneys). These tumors produce cortisol on their own, ignoring the normal signals from the pituitary gland. The tumor can be benign (an adenoma) or malignant (a carcinoma), and the distinction matters for treatment and prognosis. Malignant adrenal tumors can spread to the liver, lungs, or nearby blood vessels.
Because the tumor-bearing adrenal gland floods the body with cortisol, the pituitary gland actually dials back its signaling hormone. This causes the opposite, healthy adrenal gland to shrink from disuse. That asymmetry is one way veterinarians distinguish adrenal tumors from pituitary-driven disease on ultrasound imaging.
Iatrogenic Cushing’s: Caused by Medications
Dogs that take steroid medications (such as prednisone or dexamethasone) for allergies, autoimmune conditions, or inflammatory diseases can develop all the same symptoms as dogs with tumors. This happens because the medication is essentially synthetic cortisol. When given at high doses or for extended periods, it mimics the hormonal excess that a tumor would create. The clinical signs are identical: increased thirst, a distended belly, hair loss, and panting.
The important difference is that iatrogenic Cushing’s is reversible. Gradually tapering the steroid medication under veterinary supervision allows the body’s cortisol levels to normalize. Stopping steroids abruptly, however, is dangerous because the adrenal glands may have partially shut down during treatment and need time to resume normal function.
Why Certain Dogs Are at Higher Risk
Cushing’s disease is overwhelmingly a condition of middle-aged and older dogs, with most diagnoses occurring after age 6. Certain breeds develop it at notably higher rates, including Poodles, Dachshunds, Boston Terriers, Boxers, Beagles, and several terrier breeds. Small-breed dogs are more likely to develop the pituitary form, while larger breeds have a somewhat higher proportion of adrenal tumors, though both types occur across all sizes.
The breed predisposition suggests a genetic component, though no single gene has been identified as the cause. It’s more likely that inherited traits affecting pituitary gland regulation or adrenal gland sensitivity accumulate in certain breed lines over generations.
What Excess Cortisol Does to the Body
Cortisol is not inherently harmful. In normal amounts, it helps regulate metabolism, blood sugar, inflammation, and the immune system. The problem in Cushing’s disease is that the body is bathed in cortisol constantly, at levels far higher than normal. This chronic excess drives every visible symptom.
The characteristic potbelly that many owners notice first comes from two sources: cortisol causes fat to redistribute toward the abdomen, and it weakens the abdominal muscles so they can no longer hold the organs snugly in place. The liver also enlarges because cortisol stimulates it to store extra glycogen, pushing the belly out further.
Skin and coat changes happen because cortisol suppresses the growth cycle of hair follicles and breaks down the proteins that give skin its thickness and elasticity. Dogs develop symmetrical hair loss (often on the flanks and belly), thin skin that bruises easily, and sometimes dark patches of hyperpigmentation. Wounds heal slowly because cortisol dampens the inflammatory response that kick-starts tissue repair.
Increased thirst and urination, often the earliest signs owners notice, occur because cortisol interferes with the kidney’s ability to concentrate urine. Some dogs go from normal water intake to drinking two or three times their usual amount seemingly overnight. Increased appetite follows a similar pattern, as cortisol stimulates hunger signals in the brain.
Atypical Cushing’s and Sex Hormone Imbalances
In some dogs, standard cortisol tests come back normal even though the dog has textbook Cushing’s symptoms. These cases are sometimes called “atypical Cushing’s” and involve overproduction of other adrenal hormones, particularly sex hormone precursors like progesterone and androstenedione. The adrenal glands produce these alongside cortisol, and when the gland is overstimulated or contains a tumor, all of its hormonal outputs can increase. Diagnosing atypical cases requires testing a broader panel of adrenal hormones beyond cortisol alone.
How the Cause Is Identified
Once a veterinarian suspects Cushing’s based on symptoms and initial blood work, the next step is confirming the diagnosis and determining which type is present. The preferred screening test is the low-dose dexamethasone suppression test (LDDST), recommended by the American Animal Hospital Association. Your dog receives a small injection of a synthetic steroid, and blood samples are drawn at 4 and 8 hours afterward.
In a healthy dog, the injection signals the pituitary gland to stop producing ACTH, and cortisol drops sharply. In a dog with Cushing’s, cortisol remains elevated at the 8-hour mark (typically above 1.4 mcg/dL, though labs vary). The test can also help distinguish between pituitary and adrenal causes: if cortisol dips at the 4-hour mark but rises again by 8 hours, or drops to less than 50% of baseline at either time point, it suggests a pituitary origin. Dogs with adrenal tumors typically show no suppression at all.
Abdominal ultrasound is the other key tool. It allows veterinarians to visualize the adrenal glands directly, checking whether both are enlarged (pointing to pituitary disease) or whether one is enlarged with a visible mass while the other has shrunk (pointing to an adrenal tumor). Advanced imaging like CT or MRI can be used to evaluate pituitary tumor size when surgery or radiation is being considered.