Diagnosing Cushing’s disease in dogs requires a combination of clinical signs, blood work, and specialized hormone tests. There is no single test that confirms it on its own. Most veterinarians follow a step-by-step process: first recognizing the symptoms, then running screening blood tests, followed by one or more hormone-specific tests, and finally imaging to pinpoint the cause. The whole workup can take days to weeks depending on results.
Recognizing the Signs That Prompt Testing
Cushing’s disease typically shows up in middle-aged to older dogs, and it develops slowly enough that early signs are easy to miss. The hallmark symptoms are increased thirst, increased urination, and increased appetite. Dogs often become less active, pant excessively, and develop a distended “pot-bellied” abdomen. Skin changes are also common: thinning or fragile skin, hair loss, and recurrent skin infections.
Veterinarians generally want to see at least two of these clinical or biochemical abnormalities before pursuing Cushing’s-specific testing. A single symptom like increased thirst has too many other possible explanations. Routine blood work often reveals clues as well, particularly an elevated liver enzyme called alkaline phosphatase (ALP), high cholesterol, or a dilute urine sample. These findings, combined with the physical signs, are what raise suspicion enough to move to the next stage.
The Low-Dose Dexamethasone Suppression Test
The low-dose dexamethasone suppression test (LDDST) is the preferred first-line diagnostic test recommended by the American Animal Hospital Association. It works by giving your dog a small injection of a synthetic steroid and then measuring how the adrenal glands respond over eight hours.
Here’s what happens: the vet draws a baseline blood sample, gives the injection, then collects blood again at four hours and eight hours. In a healthy dog, the synthetic steroid signals the body to stop producing its own cortisol, so levels drop. In a dog with Cushing’s, cortisol stays elevated because the feedback loop is broken. An eight-hour cortisol level above 1.4 mcg/dL is consistent with a Cushing’s diagnosis. If it falls between 1.0 and 1.39 mcg/dL and clinical suspicion is still high, the vet may repeat the test in two to three months or move to a different test.
The LDDST has a sensitivity of about 97%, meaning it catches nearly all dogs that truly have the disease. Its specificity is lower, around 67%, so some dogs without Cushing’s will test positive. That’s why the clinical picture matters so much. To minimize stress-related interference, some clinics hospitalize the dog for at least a day before running the test so the dog can settle in. Dogs are not fasted during the test.
The ACTH Stimulation Test
The ACTH stimulation test takes a different approach. Instead of trying to suppress cortisol, it stimulates the adrenal glands with a synthetic hormone and measures how much cortisol they produce in response. A blood sample is drawn before the injection and again about an hour later. Dogs with Cushing’s typically produce an exaggerated cortisol response.
This test is less sensitive than the LDDST for pituitary-dependent Cushing’s (the most common form), catching about 80 to 83% of cases. For adrenal tumor-based Cushing’s, sensitivity drops further to 57 to 63%, so a negative result doesn’t rule it out. However, the ACTH stimulation test has one unique strength: it’s the gold standard for diagnosing iatrogenic Cushing’s, the form caused by long-term steroid medications. It’s also used later to monitor treatment once a dog starts medication.
Veterinarians often turn to this test when the LDDST doesn’t support a diagnosis but clinical suspicion remains high, or when they need to distinguish medication-induced Cushing’s from the naturally occurring form.
Urine Cortisol-to-Creatinine Ratio
The urine cortisol-to-creatinine ratio (UCCR) is a simpler screening option with one major advantage: you collect the urine sample at home, which eliminates the stress of a clinic visit that can artificially raise cortisol levels. It measures how much cortisol your dog is excreting over time relative to urine concentration.
This test is highly sensitive (over 90% in most studies) but poorly specific, meaning it produces a lot of false positives, especially in dogs that urinate frequently for other reasons. Its best use is ruling Cushing’s out. If the UCCR comes back normal, Cushing’s is unlikely. But a high result doesn’t confirm the diagnosis on its own, and your vet will still need to follow up with an LDDST or ACTH stimulation test.
Why Other Illnesses Complicate Testing
One important wrinkle: non-adrenal illnesses can cause false positives on any of these adrenal function tests. A dog that’s systemically ill, whether from an infection, uncontrolled diabetes, or another condition, may show elevated cortisol that has nothing to do with Cushing’s. Michigan State University’s veterinary diagnostic lab specifically advises against testing for Cushing’s while a dog is sick with something else. If your dog has diabetes, for example, the diabetes should be regulated first before pursuing Cushing’s testing. Otherwise, the results may be unreliable and lead to unnecessary treatment.
Figuring Out the Cause: Pituitary vs. Adrenal
Once Cushing’s is confirmed, the next step is determining whether a pituitary gland tumor or an adrenal gland tumor is responsible. This matters because treatment differs significantly. About 80 to 85% of cases are pituitary-dependent, meaning a small tumor in the brain is overproducing the hormone that drives the adrenal glands. The remaining cases involve a tumor on one of the adrenal glands themselves.
The LDDST can sometimes provide clues during the initial test. If there’s evidence of “partial suppression” at the four-hour mark (cortisol dips, then rises again by eight hours), this pattern points toward pituitary-dependent disease. But not all results are that clear-cut.
Measuring endogenous ACTH levels in the blood helps further. Dogs with pituitary-dependent Cushing’s tend to have elevated ACTH because the pituitary tumor keeps producing it. Dogs with adrenal tumors typically have low ACTH because the tumor operates independently and the excess cortisol it produces suppresses the pituitary.
The Role of Imaging
Abdominal ultrasound is a key tool for visualizing the adrenal glands directly. Veterinarians look at the size, shape, and symmetry of both glands. In pituitary-dependent Cushing’s, both adrenal glands are usually enlarged because they’ve been overstimulated. With an adrenal tumor, one gland is typically enlarged while the other appears normal or shrunken.
The commonly cited cutoff for adrenal gland enlargement is a maximum diameter of 0.74 cm, but research shows this threshold may not be appropriate for small to medium breed dogs. Adrenal gland size correlates with body weight, so a measurement that’s normal for a Labrador could be abnormal for a Yorkie. The caudal pole thickness (the width of the tail end of the gland) measured in a lengthwise view is considered the most reliable and consistent measurement. Your vet should interpret the ultrasound findings in context of your dog’s size.
For suspected pituitary tumors, advanced imaging like CT or MRI of the brain can identify the tumor’s size, which influences treatment decisions and prognosis.
What the Diagnostic Timeline Looks Like
The path from suspicion to confirmed diagnosis typically unfolds over several visits. Initial blood work and a physical exam may happen at the first appointment. If results raise suspicion, a hormone test like the LDDST is scheduled, which requires an all-day visit. Lab fees for cortisol testing generally run between $60 and $80 for the hormone panels alone, though total costs are higher when you factor in the office visit, blood draws, and any additional tests. An abdominal ultrasound adds to the expense and may require referral to a specialist depending on your clinic’s equipment.
If results are borderline or a concurrent illness is present, your vet may recommend waiting two to three months and retesting. The entire process from first suspicion to definitive diagnosis can take anywhere from a few days to several months, depending on how straightforward the results are and whether other health issues need to be addressed first.