Signs, Symptoms, and Diagnosis of Feline Hyperthyroidism 🐱

By Dr Duncan Houston, BVSc

Hyperthyroidism is the most common hormonal disorder in cats. Since emerging in the 1980s, it has become increasingly prevalent—particularly in senior cats. With early detection and proper treatment, cats can maintain excellent quality of life. Here's your 2025 guide to spotting, testing, and understanding feline hyperthyroidism with Dr Duncan Houston. 🧪

🔍 What Is Thyroid Hormone?

The thyroid glands, located on either side of the trachea, produce a hormone called thyroxine (T4). While T4 is inactive, body tissues convert it to T3 (triiodothyronine), the active form that controls metabolic rate—essentially the "volume dial" for how fast cells operate. Every organ system is influenced by thyroid hormones. 📊

Veterinarians typically measure T4 levels to assess thyroid activity. An elevated T4 is the primary diagnostic marker for hyperthyroidism. 🧬

📈 Classic Signs of Hyperthyroidism in Cats

While symptoms vary, several hallmark signs make feline hyperthyroidism recognizable:

• 📉 Weight loss despite a ravenous appetite
• 💧 Increased thirst and urination
• 😾 Restlessness, hyperactivity, or demanding behavior
• 🤢 Intermittent vomiting and/or diarrhea
• 🚽 Inappropriate urination around the home

Often, one or both thyroid glands will be enlarged and palpable during a physical exam. Hyperthyroidism most commonly affects cats aged 12–13 years. 🧓🐱

🩺 How Is Hyperthyroidism Diagnosed?

The primary tool for diagnosis is a comprehensive blood panel including thyroid levels (T4). Elevated T4 confirms the diagnosis. Additionally, liver enzymes may be mildly elevated due to cortisol or metabolic stress. 🧪

🟡 What If T4 Levels Are Borderline?

Borderline T4 levels can occur in early hyperthyroidism or when another illness suppresses thyroid hormones. Further tests include:

• T3 Suppression Test – The cat receives T3 pills for 2 days; in healthy cats, T4 drops. In hyperthyroid cats, it doesn’t. Requires pilling and return visit.
• Free T4 by Equilibrium Dialysis – Measures unbound T4, less affected by non-thyroid illnesses. Highly accurate but may require outside lab processing.
• Nuclear Scintigraphy – Imaging shows thyroid gland size/activity but is only available at specialized facilities.

🧠 What Causes Feline Hyperthyroidism?

Most cases (96–98%) are due to benign adenomatous hyperplasia—non-cancerous growth of thyroid tissue. This condition leads to the overproduction of T4. 🔬

🧬 Risk Factors:

• Age – Most cats are 12–13 years old at diagnosis
• Breed – Siamese and Himalayans have lower incidence
• Canned Food – Especially pop-top or fish-flavored varieties. These may increase risk 2–3x.
• Indoor Lifestyle – Linked with higher diagnosis rates
• Chemical Exposure – Possible links to BPA, flame retardants (PBDEs), and pollutants from litter or home cleaners

🔬 Environmental Chemical Theories

While BPA from canned linings and PBDEs from furniture have been studied, no conclusive link has been found. However, indoor cats’ higher diagnosis rate suggests increased environmental exposure and veterinary screening. 🏠

🫀 Why Treat Hyperthyroidism?

Even if your cat seems fine, untreated hyperthyroidism can cause:

• ❤️ Heart disease and potential heart failure
• 🧠 High blood pressure → sudden blindness or stroke
• 💪 Severe muscle wasting
• 🚽 Persistent vomiting or diarrhea

Treatment protects your cat’s heart, kidneys, eyes, and overall comfort. Left unchecked, it can be fatal. 🆘

🧬 Summary of Key Points

• ✅ Most common endocrine disorder in cats, especially seniors
• 📊 T4 blood test is the primary diagnostic tool
• 😺 Symptoms include weight loss with appetite, vomiting, restlessness
• 🧬 Caused by benign overgrowth of thyroid tissue
• 🥫 Canned foods (especially fish/pop-top types) may increase risk
• 👨‍⚕️ Further testing (Free T4, T3 suppression, imaging) helps in borderline cases
• 💡 Timely treatment can prevent life-threatening complications

📱 Ask A Vet Support

If you're concerned your cat might be hyperthyroid—or managing a diagnosed cat—use the Ask A Vet app for:

• 🧪 Real-time vet consultations about T4 levels and next steps
• 📤 Lab result uploads and expert feedback
• 💬 Ongoing management support and treatment advice
• 🛑 Prevention planning and environmental guidance

With early detection, expert advice, and proactive care, hyperthyroid cats can live long, happy lives. 💖🐾

🏁 Final Takeaway

Feline hyperthyroidism is treatable and manageable with the right diagnostic steps. Watch for signs, test early, and stay proactive. Partner with your vet and leverage the Ask A Vet app for full support—because your cat deserves the best care, every step of the way. 🐱🩺