Diagnosing Cushing’s disease in dogs requires a combination of recognizing physical signs, running blood and urine screening tests, and then using more specific hormone tests to confirm the condition and identify its cause. There is no single test that definitively diagnoses Cushing’s on its own, so veterinarians follow a step-by-step process that starts broad and narrows down.
Signs That Prompt Testing
Most dogs with Cushing’s disease are middle-aged or older, and the symptoms develop gradually enough that owners sometimes dismiss them as normal aging. The most obvious early signs are drinking far more water than usual, urinating more frequently (including accidents in the house), and having a noticeably increased appetite. Dogs often develop a characteristic potbelly from weakened abdominal muscles and fat redistribution, and they may pant heavily even at rest.
Skin changes are another hallmark. Many dogs lose hair in a symmetrical pattern on both sides of the body, while the head and legs stay unaffected. The skin itself can become thin, bruise easily, or develop blackheads, dark patches, or hard calcium deposits. Recurring skin infections and urinary tract infections are common because excess cortisol suppresses the immune system. If your dog has several of these signs together, your vet will likely recommend screening for Cushing’s.
Routine Bloodwork and Urinalysis
Before running hormone-specific tests, your vet will typically start with standard blood panels and a urinalysis. These don’t confirm Cushing’s, but they reveal patterns that raise suspicion. Dogs with Cushing’s often show elevated liver enzymes (particularly one called alkaline phosphatase), high cholesterol, high blood sugar, and dilute urine. If these results line up with the physical signs, the next step is targeted endocrine testing.
The Urine Cortisol-to-Creatinine Ratio
This is the simplest screening test and one you can collect at home. You gather a morning urine sample from your dog and bring it to the clinic, where the lab measures how much cortisol is being excreted relative to creatinine (a waste product that helps standardize the measurement). The test is highly sensitive, meaning it catches nearly all dogs that truly have Cushing’s. However, it’s not very specific: stress, other illnesses, and certain medications can also elevate the ratio. A normal result makes Cushing’s very unlikely and effectively rules it out. An elevated result means more testing is needed but doesn’t confirm the diagnosis on its own.
Low-Dose Dexamethasone Suppression Test
The low-dose dexamethasone suppression test (LDDS) is considered the preferred first-line diagnostic test. It has high sensitivity and can sometimes help identify the underlying cause of Cushing’s in the same step.
Here’s what happens: your dog receives a small injection of dexamethasone, a synthetic steroid. In a healthy dog, this signals the brain to stop stimulating cortisol production, and blood cortisol levels drop. In a dog with Cushing’s, that feedback loop is broken, so cortisol stays elevated. Blood is drawn before the injection and again at 4 hours and 8 hours afterward. If the 8-hour cortisol level remains above 1.4 mcg/dL, the result is consistent with Cushing’s. Values between 1.0 and 1.39 mcg/dL are considered borderline; if clinical suspicion is still high, your vet may recommend repeating the test in two to three months or moving to a different test.
The test does require your dog to stay at the clinic for most of the day. Dogs don’t need to be fasted beforehand in most cases, but your vet will give you specific instructions about withholding any medications that could interfere with results.
ACTH Stimulation Test
The ACTH stimulation test is shorter and simpler but less sensitive than the LDDS test, which is why it’s generally not recommended as the first choice for diagnosis. It’s most useful in two specific situations: when your vet suspects iatrogenic Cushing’s (caused by long-term steroid medications) or when monitoring treatment once a dog is already on medication.
Your dog receives an injection of synthetic ACTH, the hormone that tells the adrenal glands to produce cortisol. Blood is drawn before and about one hour after the injection. In a healthy dog, cortisol rises to a moderate level. In a dog with Cushing’s, the adrenal glands overreact, pushing post-stimulation cortisol above 22 mcg/dL. Results between 18 and 22 mcg/dL fall into a gray zone.
For dogs that developed Cushing’s-like symptoms from steroid medications rather than from a tumor, this test shows the opposite pattern: the adrenal glands barely respond at all, producing a flat or near-flat cortisol line. That’s because prolonged external steroids cause the adrenal glands to shrink from disuse.
Pituitary Tumor vs. Adrenal Tumor
Once Cushing’s is confirmed, the next question is what’s causing it. About 80 to 85 percent of cases are pituitary-dependent, meaning a small tumor in the brain’s pituitary gland is overproducing the hormone that drives cortisol release. The remaining 15 to 20 percent are caused by a tumor on one of the adrenal glands themselves. The distinction matters because treatment approaches differ significantly.
The LDDS test can sometimes point toward the cause. If cortisol drops below 1.4 mcg/dL at the 4-hour mark but rebounds by 8 hours, that pattern suggests a pituitary tumor. If cortisol stays stubbornly elevated at both time points, the cause could be either type, and further testing is needed.
Abdominal Ultrasound
Ultrasound is one of the most practical tools for telling pituitary from adrenal Cushing’s. A skilled ultrasonographer measures the width of each adrenal gland, focusing on the back end (caudal pole). Normal width depends on the dog’s size: up to about 0.54 cm for dogs under 10 kg, up to 0.68 cm for dogs between 10 and 30 kg, and up to 0.8 cm for dogs over 30 kg.
In pituitary-dependent Cushing’s, both adrenal glands are enlarged because both are being overstimulated by the brain. Bilateral enlargement with caudal pole widths exceeding 0.8 cm is a typical finding. In adrenal-dependent Cushing’s, one gland is visibly enlarged (often housing a mass) while the opposite gland is small or normal because it’s been suppressed. Masses larger than 2 cm raise concern for malignant tumors such as adrenal carcinomas.
Ultrasound also lets your vet check for other complications of Cushing’s, like gallbladder changes or liver enlargement, and can reveal whether an adrenal mass has invaded nearby blood vessels.
Additional Differentiation Tools
When the LDDS test and ultrasound don’t give a clear answer, vets can measure endogenous ACTH levels in the blood. In pituitary-dependent Cushing’s, ACTH is normal to elevated because the pituitary tumor keeps producing it. In adrenal-dependent Cushing’s, ACTH is very low because the adrenal tumor produces cortisol independently, and the brain’s pituitary gland shuts down its own ACTH output in response. This blood test requires careful sample handling (the hormone degrades quickly), so it’s typically sent to a specialty lab.
Advanced imaging like CT or MRI of the brain can directly visualize a pituitary tumor and measure its size. This is most relevant when radiation therapy is being considered or when a dog is showing neurological symptoms that suggest a large pituitary mass pressing on surrounding brain tissue.
Why Diagnosis Can Take Time
Cushing’s disease is one of the trickier conditions to pin down in veterinary medicine. No single test is perfect, results can fall into gray zones, and other illnesses (diabetes, kidney disease, liver disease) can mimic or complicate the picture. Stress alone can elevate cortisol enough to produce a false positive on screening tests. Your vet may need to repeat tests weeks or months apart, especially if early results are borderline.
The key principle veterinarians follow is to test only when clinical signs genuinely support the diagnosis. Running Cushing’s tests on a dog that doesn’t have compatible symptoms leads to confusing, unreliable results. If your dog has several of the classic signs, the most straightforward path is an LDDS test followed by an abdominal ultrasound, which together can both confirm the disease and identify whether a pituitary or adrenal problem is driving it.