🧬 Malignant Thyroid Tumors in Dogs and Cats – A 2025 Vet’s Guide to Diagnosis, Imaging, and Treatment

By Dr Duncan Houston, BVSc

📍 Meet the Thyroid Glands

Each pet has two thyroid glands located in the throat, one on either side of the windpipe just behind the larynx. These glands produce thyroid hormones—primarily T4 (thyroxine), which is converted by body tissues into T3 (triiodothyronine), the active form that regulates metabolism. 🧠⚙️

This article focuses on thyroid malignancy—a serious but manageable condition that affects dogs and cats differently. Let’s look at each species in turn. 🐾

🐶 Malignant Thyroid Tumors in Dogs

📉 Hormone Levels Often Remain Normal

Most dogs with thyroid tumors are neither hypothyroid nor hyperthyroid. In fact, the tumor may be completely “silent” hormonally. The condition is often discovered by noticing a lump in the neck. 📦

🧬 Typical Presentation

• 📆 Average diagnosis age: 9 years
• 🐾 No gender predisposition
• 🐕 Breeds at higher risk: Boxer, Beagle, Golden Retriever
• 📊 ~20% of dogs with thyroid carcinoma show hyperthyroid signs

🔍 Initial Assessment

If a neck lump is detected, the first steps include:

• 🧪 Bloodwork + urinalysis (including T4 levels)
• 🩻 Chest x-rays to check for lung metastasis

🔬 Diagnostic Methods

1. Fine Needle Aspirate (FNA)

• 📈 Identifies thyroid tissue in ~90% of cases
• ❌ Can’t reliably distinguish benign vs malignant

2. Surgical Biopsy

• ✅ Most accurate if the tumor is resectable
• ⚠️ Tumors are often highly vascular and invasive

📊 Malignancy Rate

🐶 Approximately 87% of canine thyroid tumors are malignant.

📡 Imaging Options

• 🩺 Ultrasound: Evaluates depth and surgical feasibility
• ☢️ Scintigraphy: Uses radioisotopes to detect thyroid tissue elsewhere in the body
• 🧲 MRI: High-resolution soft tissue imaging, requires anesthesia

Metastasis rate: 16–38% of dogs have spread at diagnosis, typically to local lymph nodes or lungs. 🎯

💊 Treatment Options for Dogs

1. 🛠️ Surgery

Best for freely movable, localized tumors. Studies show:

• 📈 Median survival time: 802 days
• 📊 77% of dogs survived >500 days

2. 🔆 External Beam Radiation

• 📅 Typically 3 sessions/week
• 📈 Median survival: ~24.5 months
• 😶 Temporary side effects: hoarse voice, cough, swallowing issues

3. ☢️ Radioiodine Therapy (Iodine-131)

• 🎯 Treats primary tumor + metastases
• 🛏️ Dog must stay in radioactive isolation temporarily
• 📈 Median survival: ~30 months
• 💊 Most dogs need thyroid supplements after therapy

4. 🧬 Adjunctive Therapies

• Toceranib (a tyrosine kinase inhibitor)
• NSAIDs may reduce tumor progression

🐱 Malignant Thyroid Tumors in Cats

😼 Most Cats Are Hyperthyroid

• 📊 95% of feline thyroid tumors are benign
• ⚠️ Only ~5% are malignant

🧪 How to Identify Malignancy

Scintigraphy is the preferred method, especially if:

• 💊 Medical treatment or radiotherapy fails to control hyperthyroidism
• 📈 Tumor seems larger or more invasive than expected

🔍 Treatment Challenges

Cats with non-hyperthyroid tumors are harder to diagnose. In these cases, a biopsy or advanced imaging may be required. 🧬

☢️ Radioiodine Therapy

• ✅ Effective for both benign and malignant tumors
• 📈 Median cancer survival: ~3 years
• ⚠️ Requires sufficient tumor iodine uptake

Other Options

• 🛠️ Surgery (rarely used today)
• 🔆 External beam radiation (if radioiodine is not feasible)

📲 Ask A Vet for Tumor Diagnosis Support

If your dog or cat has a thyroid lump or is showing signs of hormone imbalance, our team can help interpret tests, review scan results, and discuss treatment plans. 💬

Connect via the Ask A Vet App or visit AskAVet.com for expert support on thyroid tumors in pets. 🐾

✅ Summary: Key Takeaways

• 📦 Canine thyroid tumors are often nonfunctional, but usually malignant
• 📈 87% of thyroid masses in dogs are carcinomas
• 🧪 Feline thyroid tumors are usually benign but malignant ones can mimic hyperthyroidism
• ☢️ Radioiodine is effective for both species and tumor types
• 🧬 Imaging and biopsy help determine invasiveness and metastasis

💬 Final Words from Dr Duncan Houston

Malignant thyroid tumors are serious, but with advanced imaging and targeted therapies like radioiodine or surgery, many pets live for years post-diagnosis. Whether you're navigating diagnostics or post-op care, we’re here to support you. 🐶🐱💙

Need help interpreting scans or choosing a treatment? Use the Ask A Vet App or visit AskAVet.com for guidance tailored to your pet. 🐾📱