Testing a dog for Cushing’s disease typically involves blood tests that measure how your dog’s body produces and regulates cortisol, the main stress hormone. No single test confirms the diagnosis on its own. Your vet will usually start with screening bloodwork and clinical signs, then move to one or two specific hormone tests, and potentially imaging to pinpoint the cause.
Why Testing Takes Multiple Steps
Cushing’s disease (hyperadrenocorticism) means your dog’s body is producing too much cortisol, either because of a tiny tumor on the pituitary gland in the brain (about 80-85% of cases) or a tumor on one of the adrenal glands near the kidneys. The symptoms, like excessive drinking, frequent urination, a pot-bellied appearance, hair loss, and increased appetite, overlap with other conditions common in older dogs. That overlap is why your vet won’t diagnose Cushing’s from symptoms alone.
The diagnostic process has two phases: first, confirming that cortisol levels are abnormally high, and second, figuring out whether the pituitary gland or an adrenal gland is responsible. The answer to that second question determines the treatment path.
Initial Screening: Bloodwork and Urine
Before jumping to specialized hormone tests, most vets run a standard blood panel and urinalysis. Dogs with Cushing’s frequently show elevated liver enzymes (particularly one called alkaline phosphatase), high cholesterol, dilute urine, and sometimes elevated blood sugar. These findings don’t prove Cushing’s, but they help your vet decide whether the more involved hormone tests are warranted.
A urine cortisol-to-creatinine ratio is sometimes used as a quick screening tool. You collect a morning urine sample at home (which avoids stress-related cortisol spikes from visiting the clinic), and the lab compares cortisol levels against a baseline marker. This test has 100% sensitivity, meaning it catches virtually every dog with Cushing’s. However, its specificity is only about 22%, so a large number of dogs without Cushing’s will also test positive. A normal result effectively rules out the disease, but an abnormal result simply means more testing is needed.
The ACTH Stimulation Test
This is one of the two main confirmatory tests. Your vet draws a baseline blood sample, injects a synthetic version of the hormone that normally tells the adrenal glands to release cortisol, then draws a second blood sample exactly 60 minutes later. The test measures how aggressively the adrenal glands respond to that signal.
In a healthy dog, cortisol rises to a moderate level after the injection, typically staying below a reference ceiling of around 8 micrograms per deciliter. Dogs with Cushing’s produce an exaggerated response, with cortisol climbing well above that threshold. The whole process takes about 90 minutes in the clinic.
The ACTH stimulation test has a sensitivity of about 95%, meaning it correctly identifies the vast majority of dogs who actually have the disease. Its real strength is specificity: 91% of dogs without Cushing’s will correctly test negative. That gives it a strong positive predictive value, meaning a positive result is fairly reliable. Your vet may prefer this test if your dog is also being monitored for treatment response later, since it’s the standard follow-up test once treatment begins.
Preparing for the Test
Ideally, your dog should fast overnight, and the test is performed in the morning between 8 and 10 a.m. to align with the body’s natural cortisol rhythm. Let your vet know about any medications your dog takes, especially steroids (including steroid eye drops or ear medications), as these can interfere with results.
The Low-Dose Dexamethasone Suppression Test
The LDDS test works on the opposite principle. Instead of stimulating cortisol production, your vet gives a small dose of a synthetic steroid (dexamethasone) that should temporarily shut down cortisol production in a healthy dog. Blood is drawn before the injection and again at 4 and 8 hours afterward.
In a normal dog, cortisol drops significantly after receiving dexamethasone because the brain recognizes the external steroid and tells the adrenal glands to stand down. In a dog with Cushing’s, cortisol stays stubbornly high because the feedback loop is broken. A cortisol level that fails to suppress below about 1.4 micrograms per deciliter at the 8-hour mark is considered consistent with Cushing’s.
The LDDS test has similar sensitivity to the ACTH stimulation test (about 96%) but lower specificity (70%). That means it’s slightly better at catching true cases but more likely to produce false positives, particularly in dogs that are stressed or dealing with another serious illness. Its positive predictive value is about 76%, compared to 91% for the ACTH stim test. The tradeoff is that the LDDS test can sometimes provide clues about whether the problem is pituitary or adrenal based on the pattern of cortisol suppression at the 4-hour versus 8-hour marks.
This test requires your dog to stay at the clinic for most of the day. The same medication precautions apply: the test doesn’t work properly if your dog is already taking medications that suppress the natural cortisol system.
Figuring Out the Cause
Once Cushing’s is confirmed, your vet needs to determine whether a pituitary tumor or an adrenal tumor is driving the excess cortisol. This matters because the treatments are different.
Abdominal Ultrasound
Ultrasound is the most common next step and is highly effective. A 2021 study found that ultrasound detected enlarged adrenal glands in 95.6% of dogs with confirmed Cushing’s when gland size was compared against weight-based reference ranges. Normal adrenal gland size varies by body weight. For example, in dogs weighing 10 to 20 kg (roughly 22 to 44 pounds), the left adrenal gland should measure no more than about 6.4 mm and the right no more than 7.5 mm.
If both adrenal glands are symmetrically enlarged, that points toward a pituitary cause, since both glands are being overstimulated equally by excess signaling from the brain. If one gland is noticeably larger than the other, or if there’s a visible mass on one gland, an adrenal tumor is more likely. About 56% of dogs with Cushing’s show a pattern consistent with pituitary disease on ultrasound, while about 40% show asymmetric enlargement that warrants closer investigation.
Endogenous ACTH Measurement
This blood test measures the level of the natural signaling hormone (ACTH) that the pituitary gland sends to the adrenal glands. If ACTH is high or normal, the pituitary is overproducing the signal. If ACTH is very low, the adrenal gland is acting on its own and the pituitary has appropriately gone quiet. Normal dogs have ACTH levels between about 13 and 46 pg/ml.
The catch is that this test requires extremely careful sample handling. ACTH breaks down quickly in blood that isn’t immediately chilled and processed, which can produce falsely low readings. Not every general practice has the equipment to handle the sample correctly, so your vet may send it to a reference lab or recommend the ultrasound approach instead.
High-Dose Dexamethasone Suppression Test
This test follows the same logic as the low-dose version but uses a larger steroid dose. The idea is that a pituitary tumor will partially respond to the stronger signal and allow some cortisol suppression (50% or more from baseline), while an adrenal tumor won’t respond at all. Its sensitivity for correctly identifying pituitary-dependent Cushing’s is about 81%, with a specificity of roughly 67%. Because it’s not definitive in many cases, vets often rely on ultrasound and ACTH measurement instead.
When Results Are Unclear
Borderline results are not unusual. Dogs that are sick with another condition, highly stressed, or on certain medications can produce cortisol patterns that mimic Cushing’s. If initial test results fall in an equivocal range, the typical recommendation is to retest after several months rather than immediately starting treatment. Cushing’s is a slow-progressing disease in most cases, so waiting a few months for clearer results rarely causes harm.
Your vet may also repeat a different test than the one originally used. For instance, if the LDDS test was borderline, an ACTH stimulation test might provide a clearer answer, or vice versa. In some cases, advanced imaging like an MRI of the brain can identify a pituitary tumor directly, though this is typically reserved for surgical planning rather than initial diagnosis.
What Testing Costs
The hormone tests themselves are relatively affordable. Reference lab fees for an ACTH stimulation test or a dexamethasone suppression test run around $60 to $80 for the cortisol measurements alone. However, your total bill will also include the office visit, the synthetic hormone or dexamethasone injection, blood draw fees, and possibly the initial screening bloodwork. Altogether, expect each round of testing to cost somewhere between $150 and $400 at most general practices, with abdominal ultrasound adding another $200 to $500 depending on your area. Specialty or emergency clinics tend to charge more.
If your dog needs advanced differentiation testing, such as an MRI or endogenous ACTH measurement through a reference lab, costs can climb further. It’s reasonable to ask your vet to outline the expected testing sequence and approximate costs before starting, so you can plan accordingly.