Hyperthyroidism in Cats: Causes, Symptoms & Treatment

Hyperthyroidism is the most common endocrine disorder in older cats. While it affects around 10% of cats over the age of 10, it can be easy to overlook or mistake for other conditions, especially because early symptoms tend to appear subtle or might even seem positive, like higher energy levels and a better appetite. But, when left untreated, feline hyperthyroidism can cause severe complications, including heart disease, high blood pressure, weight loss and more.

Knowing the warning signs and how to manage hyperthyroidism in cats can improve your pet’s quality of life. So let’s explore the causes, symptoms, treatment options, dietary strategies and when to get support from your veterinarian, especially in cases of emergency.

Understanding hyperthyroidism in cats

Hyperthyroidism in cats is a condition caused by an overproduction of thyroid hormones (T3 and T4) from the thyroid glands, which are located in the neck. As hormones, they are responsible for regulating your cat’s metabolism. An excess amount causes the body’s systems to accelerate – and sometimes dangerously so.

The most common cause is a benign tumour called an adenoma, which appears in one or both of the thyroid glands. In rare cases, hyperthyroidism can be caused by a malignant tumour known as a thyroid adenocarcinoma.

While cats of any age can technically develop thyroid problems, hyperthyroidism in cats is most commonly diagnosed in those aged 12 and over. The disease appears to have no breed or sex predilection, although environmental factors and diet can sometimes play a role.

Recognising symptoms of feline hyperthyroidism

Knowing the signs your cat has hyperthyroidism can help with early diagnosis and a better outcome. Because the thyroid regulates metabolism, many symptoms will relate to changes in your pet’s energy levels and appetite.

Some of the most common symptoms of hyperthyroidism in cats include:

• Weight loss despite increased appetite: This is the most common red flag. If your cat is eating ravenously but still losing weight, thyroid dysfunction might be at play.
• Higher levels of thirst and urination: Both of these symptoms can be an indicator of other conditions like kidney disease or diabetes, but they are frequently seen in cats with hyperthyroidism.
• Restlessness or irritability: Cats can become more vocal or difficult to settle. You might notice higher activity levels or a change in their sleep patterns.
• Vomiting and diarrhoea: Both vomiting and diarrhoea can be a result of increased gut motility.
• Unkempt or matted fur: Your cat might appear less groomed than usual or have a greasy, unkempt coat.
• Changes in the eyes: Some cats with severe disease can show signs like dilated pupils or changes in vision. While not common, hyperthyroidism in cats' eyes is a recognised phenomenon in advanced cases.

If you notice any of these signs, especially in an older cat, see your vet as soon as possible.

Diagnosing hyperthyroidism in cats

Get started with a physical examination from your vet. They might detect an enlarged thyroid gland (also known as a ‘thyroid slip’) in the neck during palpation. Whatever their usual procedure, in order to confirm cat thyroid disease, your pet will need a blood test:

• Total T4: The most common initial test. Elevated levels usually mean hyperthyroidism.
• Free T4: Sometimes used if the total T4 is borderline.
• T3 and TSH levels: Other tests that can help clarify any ambiguous results or rule out other conditions.

In some cases, imaging (e.g., thyroid scintigraphy) can help determine the size and location of the thyroid glands and their function.

Treatment options for feline hyperthyroidism

How do you treat a cat with hyperthyroidism? The good news is that there are several hyperthyroidism cat treatment options available. The best choice will depend on your cat’s age and overall health.

1. Medication

Antithyroid drugs like methimazole (which can be administered in pill form, as a liquid or a topical gel) work by inhibiting the production of thyroid hormones. It is usually the first line of treatment and can be used over the long term or as a temporary measure before more permanent treatments are used.

• Pros: Non-invasive, reversible.
• Cons: Needs daily dosing, regular blood monitoring and can cause side effects (e.g. vomiting, liver issues).

2. Radioactive iodine therapy (I-131)

Considered the gold standard for treating thyroid cat problems. Your cat will receive a single injection of radioactive iodine, which selectively destroys overactive thyroid tissue.

• Pros: Successful in over 95% of cases, no need for ongoing medication.
• Cons: Hospitalisation at a specialist facility can be expensive.

3. Surgery

Surgically removing the affected thyroid gland (thyroidectomy) can be another good option, especially when only one gland is impacted. However, be aware that this option carries the risks of anaesthesia and complications like hypocalcaemia.

4. Dietary therapy

Feeding a strictly iodine-restricted diet can limit the thyroid gland’s ability to produce hormones. Commercial options like prescription diet food for cats have been formulated for this exact purpose and can be very helpful in managing the disease.

• Pros: Non-invasive, helpful for cats who struggle to tolerate medication.
• Cons: The cat must eat this food exclusively, which means no treats or other foods.

Dietary management and nutrition

Whether used as the primary treatment or in conjunction with medication, choosing a hyperthyroidism cat diet is an important consideration. Because cats with hyperthyroidism need a balanced, iodine-controlled diet to stabilise their thyroid hormone levels, make sure you:

• Choose prescription food: Commercial diet foods, like Hill’s Prescription Diet, are formulated to meet your cat’s nutritional needs while restricting iodine intake.
• Transition gradually: Introduce the new diet over 7-10 days to avoid any potential gastrointestinal issues.
• Keep it exclusive: The diet must be fed exclusively. Other foods, treats or even scraps from the table can render it ineffective.

Many owners ask us: “What is the best cat food for hyperthyroidism?”. Your veterinarian can help guide you on this decision based on your cat’s medical history.

Prognosis and long-term management

If you’ve ever wondered if cat hyperthyroidism can be cured, the answer really depends on the treatment:

• Radioactive iodine and surgery can be curative.
• Medication and diet are management tools rather than cures, and they rely on it being a long-term commitment.

With treatment, most cats live healthy lives for several years post-diagnosis. However, you’ll want to keep everything monitored with regular blood tests for thyroid levels, as well as watching for complications like kidney disease, which can show up once thyroid function returns to normal. Medications and diet might need adjusting as well.

And just how long can a cat live with hyperthyroidism? With early detection and proper management (including any necessary rehabilitation), many cats live normal or near-normal life spans.

When to seek emergency vet care

While hyperthyroidism tends to progress gradually, it can become an emergency if it’s not treated at all. Seek immediate care if your cat shows:

• Rapid weight loss or muscle wasting.
• Panting, laboured breathing or signs of heart disease.
• Seizures, collapse or lethargy.
• Extreme vomiting or diarrhoea.
• Any combination of the above, alongside suspected cat thyroid disease.

The AREN Network has round-the-clock emergency and specialty care through veterinary medicine and internal medicine specialists. If your cat is showing signs of severe illness, contact your nearest AREN clinic immediately.

While hyperthyroidism in cats is common, the good news is that it’s also very treatable. If you notice symptoms like increased appetite, weight loss or restlessness, speak to your vet about thyroid screening.

Early detection and a solid treatment plan – whether medication, surgery, radioactive iodine or through diet – can give your cat the best chance of having a long and comfortable life. And when urgent care is needed, the AREN Network is here to help you find your closest vet emergency hospital.