🧠 Cushing’s Syndrome in Dogs – A 2025 Vet’s Guide to Classifying Pituitary vs. Adrenal Types

By Dr Duncan Houston, BVSc

🔍 Why Classifying Cushing’s Syndrome Matters

Once a dog is diagnosed with Cushing’s syndrome (hyperadrenocorticism), the next essential step is to determine which type of the disease they have—either pituitary-dependent (PDH) or adrenal tumor (AT). This classification dramatically influences treatment decisions, surgical options, prognosis, and follow-up strategy. 🩺🐾

📊 The Breakdown:

• 🔵 85% of cases = Pituitary-Dependent Hyperadrenocorticism (PDH)
• 🔴 15% of cases = Adrenal Tumor (AT)

Discriminatory testing—used to classify the disease—is performed after the initial Cushing’s diagnosis is confirmed. 🧪

🧬 Why Classification Impacts Care

• 💉 Medication protocols vary greatly between PDH and AT
• 🔪 Surgery is only applicable and very different for adrenal tumors
• 🧠 Pituitary tumors may require brain imaging, especially if neurological signs are present
• 📈 Prognosis and long-term management depend on knowing the source of cortisol overproduction

🧪 Tests to Classify Cushing’s Syndrome in Dogs

1. 💉 Low-Dose Dexamethasone Suppression Test (LDDST)

This is the same test often used to diagnose Cushing’s—but with a twist, it can also help classify the type:

• If cortisol levels suppress at 4 hours (below 50% of baseline) and then “escape” by 8 hours, it's a pituitary tumor.
• Adrenal tumors never suppress—if suppression is seen, further testing may not be needed.

Success rate: Around 60% of PDH cases can be identified this way. 🎯

2. 💉 High-Dose Dexamethasone Suppression Test (HDDST)

Similar to LDDST, but with a higher steroid dose. Here's how it helps:

• 📉 PDH cases may show suppression with high doses
• ⛔ Adrenal tumors never suppress

Limitations: Up to 30% of pituitary tumors don’t suppress even with high-dose testing, making this test unreliable as a sole classifier. ❗

3. 🧪 Endogenous ACTH Level

This test measures the ACTH hormone produced by the pituitary:

• ⬆️ High ACTH = Pituitary tumor (PDH)
• ⬇️ Low or undetectable ACTH = Adrenal tumor (AT)

Caveat: ACTH is fragile. The blood sample must be frozen immediately and stay frozen during transport—if not, results may be inaccurate. ❄️📦

4. 🖥️ Imaging (Ultrasound, MRI, or CT)

Imaging is powerful and non-invasive:

• 🧠 PDH: Both adrenal glands will be symmetrically enlarged
• ⚖️ AT: One gland is enlarged/misshapen, the other is small or atrophied

Ultrasound is the most common method—it’s affordable, widely available, and does not require general anesthesia. MRI and CT scans offer more detail but are costlier and typically need referral to a specialty clinic. 🏥

🧪 In one study, ultrasound correctly identified adrenal tumors in 86% of cases. It can also help detect malignant tumors that invade the vena cava (the body’s largest vein). 🩸

🎲 Should You Just “Play the Odds” and Assume PDH?

Since 85% of dogs with Cushing’s have PDH, it may seem reasonable to skip classification and start PDH-targeted medication. But here’s the risk:

• If the dog doesn’t respond to treatment, you won’t know if the dose is wrong or if you’re treating the wrong disease type.
• Valuable time could be lost pursuing ineffective therapies. ⏳

➡️ Bottom line: While odds favor PDH, classification is strongly recommended for effective, individualized treatment. 🧠

🐱 Classification in Cats

Though rare in cats, Cushing’s can occur. Here’s how classification differs:

1. 💉 HDDST in Cats

• PDH cats may show suppression—but only about 50% do
• No suppression = inconclusive (could be PDH or AT)

2. 🧪 Endogenous ACTH

• High = PDH
• Low = AT

Transport and freezing are also essential for feline samples. ❄️

3. 🖥️ Imaging for Cats

• PDH = Both adrenals large and similar size
• AT = One adrenal enlarged, other atrophied

Ultrasound remains the easiest, safest imaging tool for cats as well. 🐈

📱 Ask A Vet: Get Help with Testing and Classification

If your pet has been diagnosed with Cushing’s syndrome, it’s important to take the next step: determining the type. This allows your veterinarian to prescribe the right medication, plan for surgery if needed, and anticipate long-term care. 🩺

Need help interpreting your pet’s test results or deciding whether to pursue imaging? Download the Ask A Vet App for expert guidance and one-on-one veterinary support. Or visit AskAVet.com for more information. 📲🐶

✅ Summary: Key Takeaways

• 👩‍⚕️ Classification is crucial for treatment and prognosis
• 🧠 PDH is the most common (85%), but AT has different risks
• 💉 Tests include LDDST, HDDST, endogenous ACTH, and imaging
• 🖥️ Ultrasound is reliable and cost-effective for identifying adrenal tumors
• 🎯 Playing the odds is risky—classification saves time and ensures accuracy

🐾 Final Words from Dr Duncan Houston

If your dog or cat has Cushing’s syndrome, proper classification is more than a technical step—it’s the key to unlocking targeted, effective care. Whether the condition stems from a tiny pituitary tumor or a malignant adrenal mass, knowing the source helps your vet chart the right course. 🧠⚖️

And if you need a hand navigating the process, from blood tests to imaging referrals, Ask A Vet is your go-to expert team. Visit AskAVet.com or download the app today. 📱🐕‍🦺