🧠 Vet 2025 Guide: Classifying Cushing's Syndrome in Pets – Adrenal vs. Pituitary Types Explained 🐶🐱

Once a pet is diagnosed with Cushing's syndrome, the next crucial step is determining whether the condition is caused by a pituitary tumor (PDH) or an adrenal tumor (AT). This classification dramatically affects treatment plans, surgical options, and prognosis. In this comprehensive Vet 2025 guide, Dr Duncan Houston explains the tests and clinical decisions that follow a Cushing’s diagnosis. 🩺

🤔 Why Classification Matters

💡 Here’s why determining the type of Cushing’s is essential:

• 🧠 85% of cases are pituitary dependent (PDH), often involving a small pituitary tumor.
• 🧱 15% involve adrenal tumors (AT), which can be benign or malignant.
• 🧬 Treatment paths and medications differ significantly between types.
• 🔍 Imaging priorities differ—brain scans for pituitary vs. abdominal scans for adrenal tumors.
• 🧪 Prognosis and surgical approaches are condition-specific.

🐶 Classifying Cushing’s in Dogs

🔬 Low Dose Dexamethasone Suppression Test (LDDS)

This test often helps diagnose and classify at the same time. If cortisol levels drop below 50% at 4 hours and rise again by 8 hours, it strongly indicates PDH. ❗Adrenal tumors do not suppress cortisol.

✅ This test correctly classifies ~60% of PDH cases, potentially avoiding further diagnostics. 🧪

📊 High Dose Dexamethasone Suppression Test (HDDS)

This uses a higher dexamethasone dose:

• 🧠 Pituitary tumors suppress cortisol in 70–80% of cases
• 🧱 Adrenal tumors do not suppress cortisol
• ⚠️ 20–30% of pituitary cases won't suppress even with high dose

Because of this, HDDS isn't always definitive, but is helpful when LDDS results are inconclusive.

🧪 Endogenous ACTH Level

This measures the ACTH hormone in the blood:

• 📈 High ACTH = pituitary tumor (PDH)
• 📉 Low or undetectable ACTH = adrenal tumor (AT)

⚠️ This test is highly accurate, but samples must be frozen and shipped with extreme care due to ACTH’s fragility. ❄️

🩻 Imaging (Ultrasound, MRI, CT)

• 📷 PDH: Both adrenal glands are enlarged but symmetrical
• 📷 AT: One adrenal gland is enlarged or misshapen, the other is atrophied

🧠 Pituitary imaging may be needed for brain pressure assessment, while abdominal imaging checks for tumor size or vena cava invasion. Tumors >4 cm are highly suspicious for malignancy.

🎲 Playing the Odds?

Since 85% of cases are PDH, some vets begin treatment assuming pituitary origin. ⚠️ Risk: if symptoms don't improve, you won’t know if the dose is wrong or if the diagnosis was incorrect. Accurate classification from the start avoids wasted time and ineffective treatments. ⏱️

🐱 Classifying Cushing’s in Cats

🧪 High Dose Dexamethasone Suppression Test

• 📉 If cortisol suppresses: indicates pituitary tumor (PDH)
• 📈 If cortisol does not suppress: test is inconclusive (50% of feline PDH cases don’t suppress)

Despite limitations, this test is non-invasive and worth trying first. 🧪

🧬 Endogenous ACTH

As in dogs, this test is reliable but depends on flawless cold-chain transport to preserve the hormone. 🧊

• 📈 High ACTH = PDH
• 📉 Low ACTH = AT

🖥️ Imaging

• 📷 PDH: Both adrenals appear enlarged and symmetrical
• 📷 AT: One large adrenal and one tiny or invisible adrenal

🩺 Ultrasound is the easiest and most affordable imaging tool, while MRI/CT offer more detail if needed. Imaging also checks for local tumor invasion—especially into the vena cava. 🧠

🧠 Why Pituitary vs. Adrenal Matters for Treatment

• 💊 PDH (Pituitary): Managed medically—often with trilostane or mitotane
• 🔪 AT (Adrenal Tumor): Often requires surgical removal—especially if malignant
• 🧬 Prognosis and medication response differ significantly

🩺 Summary: Tests to Classify Cushing’s Syndrome

📱 Need Help with Diagnosis or Testing?

Is your pet newly diagnosed with Cushing’s? Or are you stuck trying to determine the next step? Visit AskAVet.com or download the Ask A Vet app to speak directly with veterinary experts like Dr Duncan Houston. We’ll guide you through testing options and treatment strategies—tailored to your pet’s needs. 🐶🐱🧠

🔬 Accurate diagnosis is the foundation of successful care. Don’t guess—test! 🧪🐾