Hyperthyroidism in Cats
Hyperthyroidism is a very common hormonal disease in older cats that can affect the heart, kidneys, blood pressure, and overall metabolism. Because cats often hide illness until it is advanced, early veterinary evaluation for weight loss, appetite changes, or behavior changes is especially important.
Book a vet visit soon if you suspect hyperthyroidism
Hyperthyroidism is usually not a minute-to-minute emergency, but it should not be ignored. Untreated disease can lead to high blood pressure, heart changes, weight loss, and worsening secondary problems. Seek urgent veterinary care right away if your cat has trouble breathing, sudden blindness, collapse, severe weakness, or stops eating.
Symptoms to Watch For
Weight loss despite eating well
Many affected cats lose weight even though their appetite increases.
Increased appetite
A suddenly ravenous appetite can be a classic sign.
More thirst and urination
You may notice larger clumps in the litter box or more trips to the water bowl.
Restlessness or hyperactivity
Some cats seem more vocal, agitated, or unable to settle.
Vomiting or diarrhea
Digestive upset can occur as metabolism speeds up.
Poor coat quality
The coat may look greasy, matted, or unkempt.
What Causes Hyperthyroidism
Hyperthyroidism happens when the thyroid gland produces too much thyroid hormone, causing the body to run in an overactive metabolic state. In cats, this is most often caused by benign adenomatous hyperplasia or a benign hormone-producing thyroid tumor affecting one or both thyroid lobes. Thyroid carcinoma is uncommon at initial diagnosis. The exact underlying cause is still not fully known, though age appears to play a major role and researchers have explored possible dietary and environmental contributors. Most cats diagnosed are older adults, and early signs can be subtle because cats often hide illness well.
- Usually seen in cats older than 10 years
- Most commonly caused by benign thyroid enlargement or benign tumor tissue
- Rarely caused by thyroid carcinoma
- Possible contributing factors studied include diet-related factors and chronic exposure to thyroid-disrupting chemicals
- No strong breed predisposition is consistently established, but some sources suggest lower risk in Siamese, Burmese, Persian, British Shorthair, Abyssinian, and Tonkinese cats
How It's Diagnosed
Diagnosis starts with your veterinarian reviewing your cat’s history, checking body weight and heart rate, feeling the neck for an enlarged thyroid gland, and measuring blood pressure. Bloodwork is usually used to measure thyroid hormone levels, especially total T4. Because some hyperthyroid cats can have T4 values within the normal range early in disease or when another illness is present, your vet may recommend repeat testing or additional thyroid tests such as free T4 and sometimes TSH. A chemistry panel, complete blood count, urinalysis, and kidney assessment help look for related problems or conditions that may be masked by the hyperthyroid state. In some cases, imaging or thyroid scintigraphy may be used, especially when planning surgery or looking for ectopic thyroid tissue.
- Physical exam and neck palpation for thyroid enlargement
- Blood pressure measurement
- Total T4 blood test as a common first-line test
- Possible follow-up testing with free T4, repeat T4, or TSH in unclear cases
- CBC, chemistry panel, urinalysis, and kidney evaluation to assess overall health and comorbidities
Treatment Options
Treatment is often very successful, and your veterinarian will recommend the best option based on your cat’s age, kidney function, heart status, home environment, and your ability to give long-term therapy. Radioactive iodine (I-131) is widely regarded as the preferred treatment when available because it destroys abnormal thyroid tissue and cures most cats without requiring anesthesia or lifelong medication. Methimazole is commonly used to control the disease medically and may be helpful long term or as a trial before definitive treatment, but cats need regular lab monitoring for effectiveness and side effects. Surgery can be curative in selected patients, though anesthesia and parathyroid injury are important considerations. A prescription iodine-restricted diet may help control hyperthyroidism in some cats, but it only works if the cat eats that diet exclusively and has no access to other food, treats, or prey.
- Radioactive iodine (I-131): often the treatment of choice and curative for most cats
- Methimazole: controls thyroid hormone production but does not cure the disease
- Surgery: may be appropriate for selected cats after stabilization
- Prescription iodine-restricted diet: only effective if fed exclusively
- Ongoing monitoring is important because treatment can unmask kidney disease or lead to low thyroid levels
Hyperthyroidism is most often diagnosed in older cats.
Thyroid carcinoma is uncommon; most cases are linked to benign thyroid change.
Radioactive iodine is curative in approximately 95% or more of cases.
Costs vary widely by region, facility, and treatment type, so owners should ask their veterinarian for current local estimates.
Living with Hyperthyroidism
Frequently Asked Questions
Is hyperthyroidism in cats an emergency?
Can hyperthyroidism be cured?
What is the most common sign of hyperthyroidism in cats?
Can hyperthyroidism affect the kidneys?
Can diet alone treat hyperthyroidism?
Are some cat breeds more likely to get hyperthyroidism?
Sources
- Cornell Feline Health Center: Hyperthyroidism in Cats
- Merck Veterinary Manual: Disorders of the Thyroid Gland in Cats
- VCA Animal Hospitals: Hyperthyroidism in Cats
- AAHA Selected Endocrinopathies of Dogs and Cats Guidelines (2023) — Feline Hyperthyroidism section
- Hyperthyroidism in Cats: Symptoms, Diagnosis & Treatment — SpectrumCare
Disclaimer: The information provided in this article is for educational purposes only and does not constitute veterinary medical advice. Reading this content does not establish a veterinarian-client-patient relationship. Every pet is different — always consult a licensed veterinarian before making decisions about your pet's health, diet, or care. If you'd like personalized guidance, you can talk to one of our vets. If your pet is experiencing a medical emergency, contact your local emergency animal hospital immediately.