Hyperthyroidism in Cats: Causes, Symptoms, Treatment, and Prevention

Hyperthyroidism is one of the most common hormonal disorders in middle-aged and senior cats. It occurs when the thyroid glands produce excessive amounts of thyroid hormones, leading to a faster metabolism and a range of health problems. If left untreated, hyperthyroidism can cause severe complications, including heart disease, high blood pressure, and organ damage.

This guide will cover everything cat owners need to know about hyperthyroidism, including its causes, symptoms, diagnosis, treatment options, and preventive measures.

What is hyperthyroidism in cats?

Hyperthyroidism is a condition caused by an overactive thyroid gland, which produces excessive levels of thyroid hormones, particularly thyroxine (T4). The thyroid glands, located in the neck, play a crucial role in regulating metabolism. When they become overactive, a cat’s metabolism speeds up, leading to significant weight loss and other health issues.

Hyperthyroidism is most commonly diagnosed in cats over the age of ten, with no specific breed or gender predisposition.

Causes of hyperthyroidism in cats

The exact cause of hyperthyroidism is not fully understood, but several factors may contribute to its development:

Thyroid adenoma

The most common cause of hyperthyroidism is a benign (non-cancerous) tumour of the thyroid gland, known as a thyroid adenoma. This tumour causes the gland to produce excessive thyroid hormones.

Thyroid carcinoma

In rare cases, hyperthyroidism can be caused by a malignant (cancerous) tumour of the thyroid gland, known as thyroid carcinoma.

Dietary factors

Some studies suggest that certain dietary components, such as excessive iodine or exposure to environmental chemicals in commercial cat food, may contribute to the development of hyperthyroidism.

Environmental factors

Exposure to household chemicals, flame retardants, and other environmental toxins has been linked to an increased risk of hyperthyroidism in cats.

Symptoms of hyperthyroidism in cats

Hyperthyroidism affects multiple organ systems and can cause a variety of symptoms. The severity of symptoms varies depending on how advanced the disease is.

Early symptoms

• Weight loss despite a good or increased appetite
• Increased appetite (polyphagia)
• Increased thirst and urination
• Hyperactivity or restlessness
• Vomiting or diarrhoea
• Poor coat condition or excessive grooming

Advanced symptoms

• Muscle wasting and weakness
• Rapid or irregular heartbeat (tachycardia)
• High blood pressure (hypertension)
• Panting or difficulty breathing
• Enlarged thyroid gland (visible or palpable lump in the neck)
• Behavioural changes such as aggression or nervousness

Severe complications

If left untreated, hyperthyroidism can lead to:

• Heart disease, including hypertrophic cardiomyopathy
• High blood pressure, which can cause damage to the heart, kidneys, and eyes
• Kidney disease, as increased blood flow can strain the kidneys
• Blindness due to retinal detachment caused by high blood pressure

Diagnosing hyperthyroidism in cats

Hyperthyroidism is diagnosed through a combination of clinical signs, physical examination, and laboratory tests.

Veterinary examination

A vet will assess the cat’s weight, heart rate, and overall health. They may feel for an enlarged thyroid gland in the neck.

Blood tests

• Total T4 (thyroxine) test – Measures thyroid hormone levels in the blood. Elevated levels confirm hyperthyroidism.
• Free T4 (by equilibrium dialysis) – Used for borderline cases where total T4 levels are not conclusive.
• T3 suppression test – Occasionally used if standard tests are inconclusive.

Additional tests

• Blood pressure measurement – Checks for hypertension.
• Electrocardiogram (ECG) – Assesses heart function if heart disease is suspected.
• Kidney function tests – Hyperthyroidism can mask kidney disease, so kidney function must be evaluated.

Treatment options for hyperthyroidism in cats

There are several treatment options for hyperthyroidism, and the best choice depends on the cat’s age, overall health, and owner preference.

Medication (anti-thyroid drugs)

Anti-thyroid medications, such as methimazole (Felimazole), are commonly used to control hyperthyroidism by reducing thyroid hormone production.

Pros:

• Non-invasive and relatively inexpensive
• Can be adjusted based on response

Cons:

• Requires daily administration (tablets or transdermal gel)
• Can cause side effects such as vomiting, liver damage, or blood disorders
• Does not cure hyperthyroidism, only manages symptoms

Radioactive iodine therapy

Radioactive iodine (I-131) treatment is considered the gold standard for curing hyperthyroidism. It involves a single injection of radioactive iodine, which destroys overactive thyroid cells while preserving normal tissue.

Pros:

• Permanent cure in most cases
• No need for daily medication
• Low risk of side effects

Cons:

• Requires hospitalisation for a few days due to radiation exposure
• More expensive upfront compared to medication
• Not suitable for cats with advanced kidney disease

Surgical thyroidectomy

Surgical removal of the affected thyroid gland is another treatment option.

Pros:

• Can provide a permanent cure

Cons:

• Requires general anaesthesia, which carries risks in older cats
• Potential for complications such as damage to the parathyroid glands, which regulate calcium levels
• Can be difficult if both thyroid glands are affected

Prescription diet (low-iodine food)

A low-iodine diet can be used to manage hyperthyroidism by restricting iodine intake, which is essential for thyroid hormone production.

Pros:

• Non-invasive and does not require medication
• Can be effective in controlling hormone levels

Cons:

• Must be the only food the cat eats (no treats or other diets)
• Some cats may not accept the restricted diet
• Does not cure hyperthyroidism, only manages symptoms

Managing hyperthyroidism in cats

Once treatment begins, regular monitoring is essential to ensure the condition is controlled and to detect any complications.

Regular vet check-ups

Cats receiving medication or dietary management should have blood tests every few weeks initially, then every three to six months once stabilised.

Monitoring kidney function

Hyperthyroidism increases blood flow to the kidneys, which can temporarily mask kidney disease. Once treated, kidney function may need close monitoring.

Managing high blood pressure

If a cat has high blood pressure, additional medication such as amlodipine may be required to prevent complications.

Preventing hyperthyroidism in cats

While there is no guaranteed way to prevent hyperthyroidism, certain steps may help reduce the risk:

• Feeding a balanced diet with appropriate iodine levels
• Avoiding excessive exposure to environmental toxins, such as flame retardants and pesticides
• Regular veterinary check-ups for early detection of thyroid issues

Can hyperthyroidism in cats be cured?

Hyperthyroidism can be cured with radioactive iodine therapy or surgery. Medication and diet control the disease but do not eliminate it. The best treatment option depends on the individual cat’s health and the owner’s ability to manage long-term care.

When to see a vet

Seek veterinary care if your cat shows signs of:

• Weight loss despite a good appetite
• Increased thirst and urination
• Hyperactivity or restlessness
• Vomiting or diarrhoea
• Fast or irregular heartbeat

Early diagnosis and treatment can prevent complications and improve a cat’s quality of life.

Conclusion

Hyperthyroidism is a common but manageable condition in older cats. With early detection, appropriate treatment, and regular monitoring, affected cats can live healthy, happy lives. Understanding the available treatment options and working closely with a vet will help ensure the best outcome for your cat.

If you suspect your cat may have hyperthyroidism, schedule a veterinary check-up as soon as possible. Early intervention is key to preventing severe complications and maintaining your cat’s well-being.