What Cushing's Disease Actually Is
Cushing's disease — more formally hyperadrenocorticism — is a condition in which the adrenal glands produce excessive cortisol. Cortisol is the body's primary stress hormone. In normal function, the hypothalamus releases CRH (corticotropin-releasing hormone), which tells the pituitary gland to release ACTH, which tells the adrenal glands to produce cortisol. A functioning system keeps cortisol levels in a narrow, appropriate range.
In Cushing's disease, this feedback loop breaks down. Roughly 80–85% of cases are pituitary-dependent — a benign tumor on the pituitary gland (usually a microadenoma) causes it to over-secrete ACTH, which constantly stimulates the adrenal glands. The remaining 15–20% are adrenal-dependent — caused by a tumor directly on one or both adrenal glands, which may be benign or malignant.
The practical consequence is the same in either case: chronically elevated cortisol, which over months and years causes damage across multiple organ systems. Unlike acute illness, the symptoms develop gradually — which is why both owners and vets often attribute early signs to normal aging.
Why It Appears in Senior Dogs
The median age of diagnosis is 10–12 years. This is not a coincidence. Pituitary tumors become more clinically significant as they grow — a microadenoma that has been present for years may finally reach a size where it produces enough ACTH to generate visible symptoms. Adrenal tumors also become more common with age.
Certain breeds are overrepresented: Poodles (all sizes), Dachshunds, Boxers, Boston Terriers, Labrador Retrievers, and Staffordshire Terriers have higher-than-average rates. This suggests a genetic component, though the inheritance pattern is not fully characterized. There is no meaningful prevention — it is a disease of biology, not lifestyle.
The Early Signs That Look Like Normal Aging
This is where Cushing's is sneaky. The earliest symptoms are easily mistaken for the normal slowdowns of senior years. Owners frequently notice them months or years before presenting to a vet, and vets frequently attribute them to age during the first examination.
Increased thirst and urination: The earliest and most consistent sign. If your senior dog suddenly starts waking you at night to go out, or you are filling the water bowl more frequently, this is not normal aging. Elevated cortisol impairs the kidney's concentrating ability, causing more dilute urine and compensatory increased drinking. Many owners attribute this to diabetes or kidney disease — both are on the differential, which is why vet evaluation is necessary.
Increased appetite: Cortisol is an appetite stimulant. Senior dogs that become persistently food-driven after years of moderate appetite are showing a classic Cushing's sign.
Thinning coat and skin changes: The hair on the flanks and trunk becomes brittle, dry, and thin. Symmetrical hair loss on both sides of the body — not patchy, not itchy — is characteristic. The skin itself may become darker (hyperpigmented) and feel thin and fragile. These are direct effects of cortisol on keratinocytes and hair follicle cycling.
Pot-bellied appearance: As abdominal muscles weaken and fat redistributes under the influence of cortisol, the abdomen becomes visibly distended. This is often described by owners as their dog looking "pregnant" or "bloated." Combined with thinning limbs from muscle wasting, the pot-bellied look becomes quite distinctive in moderate-to-advanced cases.
Lethargy and weakness: Chronic cortisol elevation suppresses protein synthesis and impairs muscle function. Senior dogs with Cushing's often become less interested in walks, slower to rise, and less willing to engage in activities they previously enjoyed.
How Veterinarians Diagnose It
Cushing's diagnosis is a multi-step process. No single test is definitive in all cases — which is why a structured diagnostic approach matters.
Initial blood work: A senior wellness panel often shows suggestive findings: elevated ALP (alkaline phosphatase) is present in roughly 90% of Cushing's cases — cortisol induces this liver enzyme. Elevated cholesterol, mild hyperglycemia, and low-normal lymphocytes are also common. These findings are not diagnostic on their own, but they raise the index of suspicion.
Urine cortisol:creatinine ratio (UCCR): This is the best screening test — a single urine sample can rule out Cushing's with high confidence if normal. A positive result requires further testing; a negative result essentially excludes the disease. The practical advantage: no fasting, no stress required for the sample collection, and it can be done on a routine urine sample during a regular visit.
ACTH stimulation test: The traditional confirmatory test. Blood is drawn, cortisol is measured, then synthetic ACTH is injected, and cortisol is measured again 1–2 hours later. Cushing's dogs show an exaggerated cortisol response — the stimulated level is far higher than in a healthy dog. The test is good for confirming the disease but cannot distinguish between pituitary and adrenal causes. It also misses roughly 20–25% of pituitary-dependent cases (false negatives).
Low-dose dexamethasone suppression test (LDDST): More sensitive than the ACTH stimulation test — detects approximately 95% of Cushing's cases. Dexamethasone suppresses cortisol in healthy dogs; in Cushing's dogs, the suppression fails. It also provides some differentiation: pituitary-dependent dogs show partial suppression at the 4-hour mark, while adrenal-tumor dogs show no suppression at all. The drawback: it requires a longer clinic visit and is more expensive.
Abdominal ultrasound: The key test for differentiating pituitary vs adrenal dependent Cushing's. An experienced sonographer can visualize adrenal gland size and any masses. Both glands enlarged suggests pituitary-dependent; one gland enlarged with the other atrophied suggests an adrenal tumor. Ultrasound also identifies the roughly 15–20% of adrenal cases where the tumor is potentially surgical.
The practical diagnostic pathway most vets use: screen with UCCR, confirm with ACTH stimulation, differentiate with ultrasound. This is cost-effective and sufficient for most cases.
Treatment: Medication vs Surgery
Treatment depends on the type and severity of Cushing's, the dog's overall health, and owner circumstances.
Medical management (most common): Trilostane (brand name Vetoryl) is the standard medical treatment for both pituitary-dependent and adrenal-dependent Cushing's. It works by inhibiting an enzyme (3-beta-hydroxysteroid dehydrogenase) required for cortisol synthesis. It does not cure the disease — it controls it. Dogs require lifelong daily medication and regular monitoring blood tests (ACTH stimulation, typically every 3–6 months once stable).
Side effects of trilostane are uncommon but include lethargy, reduced appetite, vomiting, and — rarely — adrenal necrosis. The risk is highest in the first weeks of treatment, which is why vet monitoring is essential early on. Selegiline (Anipryl) is an alternative medication used primarily for pituitary-dependent Cushing's; it is less effective than trilostane and used less commonly.
Surgical treatment (adrenal tumors only): Removal of an adrenal tumor (adrenalectomy) can cure adrenal-dependent Cushing's. It is a major abdominal surgery with meaningful perioperative risk, particularly given that many affected dogs are elderly. It is typically only pursued when imaging shows a discrete, non-invasive tumor and the dog is otherwise healthy. Surgery at a veterinary teaching hospital or specialty surgical center significantly improves outcomes.
Radiation therapy (pituitary tumors): Stereotactic radiosurgery (Gamma Knife or similar) can shrink pituitary tumors and reduce ACTH production. It is effective but expensive and available only at specialty centers. For dogs with large pituitary tumors causing neurological symptoms (head pressing, circling, seizures), it may be the best option.
Prognosis and Quality of Life
With appropriate medical management, dogs with Cushing's disease can live normal, comfortable lives for several more years. The median survival time after diagnosis with trilostane treatment is 2–4 years, though this is heavily influenced by the dog's age at diagnosis, other concurrent conditions, and how well the disease is controlled.
The most important thing to monitor at home: appetite and energy level. A well-controlled Cushing's dog maintains a good appetite and normal activity. If either drops significantly, the medication dose may need adjustment — or the dog may be entering an adrenal crisis (rare but acute and requires immediate vet care).
Uncontrolled Cushing's — left untreated — significantly shortens lifespan and quality of life. The chronic effects compound: diabetes mellitus develops in roughly 10–20% of untreated Cushing's dogs (cortisol is diabetogenic); hypertension causes progressive organ damage; muscle wasting leads to severe weakness; the immune suppression from chronic cortisol elevation increases infection risk. These are the stakes of untreated disease.
Concurrent Conditions to Watch For
Cushing's dogs often have other age-related conditions simultaneously. Managing Cushing's well requires being aware of these interactions.
Diabetes mellitus: As noted, cortisol is antagonistic to insulin. Dogs that develop concurrent diabetes require insulin treatment alongside their Cushing's management — a significantly more complex management picture. Dogs whose Cushing's is well-controlled may see their diabetes resolve or require less insulin. See our diabetes guide for more detail.
Kidney disease: Hypertension from Cushing's accelerates kidney damage. Senior dogs with both conditions need blood pressure monitoring and potentially antihypertensive medication alongside Cushing's treatment. Regular blood work and urine protein assessment every 3–6 months is the standard of care.
Urinary tract infections: Cushing's dogs are more susceptible due to the immunosuppressive effect of cortisol and the glucose that may be present in urine. Recurrent UTIs that don't resolve with standard antibiotics should prompt evaluation for an underlying hormonal cause.
For a broader view of managing multiple chronic conditions in senior dogs, see our senior dog chronic conditions guide.