Hyperthyroidism in cats: Symptoms, causes, treatment

While having a fast metabolism sounds like a good thing, it’s unfortunately not all that it’s “cranked up” to be, at least for our feline friends!

Hyperthyroidism is a hormonal imbalance condition that causes an increased metabolic rate in the body, affecting the health and functioning of multiple organs. It is relatively common in our beloved senior kitizens, developing in around 10% of cats over the age of ten.

If you have a feline family member, it’s worth being aware of the key signs and symptoms of hyperthyroidism, particularly since with appropriate veterinary assessment and treatment, affected cats can be managed or even permanently cured. Paw-some!

Understanding hyperthyroidism in cats

In the upper neck region, on either side of the windpipe (trachea), lie the two thyroid glands. These glands produce thyroid hormones T3 and T4, which regulate metabolic rate.

Hyperthyroidism is when one or both of the thyroid glands gradually develop enlarged nodules of abnormal tissue. These nodules are usually non-cancerous tumours known as adenomas (although rarely, they can become cancerous adenocarcinomas over time), which produce excessive amounts of T3 and T4. This causes the cat’s metabolism to go into overdrive, which negatively impacts their quality of life and may unfortunately also cause damage to organs such as the heart, kidneys and eyes.

Common signs and symptoms

Symptoms of hyperthyroidism may be subtle at first but become more severe as the condition progresses. Common signs of hyperthyroidism can include:

• Weight loss despite an increased appetite
• Increased thirst
• Increased urination
• Tummy upset symptoms (such as loose stools or intermittent vomiting)
• Changes in behaviour, such as hyperactivity, increased vocalisation, or irritability
• Reduced coat quality (appearing unkempt or greasy)
• A fast heart rate and/or abnormal heart sounds

Cats with hyperthyroidism also have a 10-25% risk of developing secondary hypertension, also known as high blood pressure.

Causes and risk factors

The exact cause of feline hyperthyroidism isn’t yet known, although possible contributing factors may include dietary imbalances in iodine or long-term exposure to thyroid-disrupting chemicals found in particular foods, water or the environment (such as BPA’s or flame retardants).

Diagnosis of hyperthyroidism

Hyperthyroidism is often suspected in a cat with a few of the symptoms listed above, and the palpation of an enlarged thyroid gland in their neck. However, to confirm the condition, your vet will need to perform special blood tests to assess their thyroid hormone function. This will most commonly involve checking their blood T4 levels, though some cats may also require additional thyroid hormone checks if their initial results don’t give a clear answer.

As well as the specific thyroid hormone assessments, your vet will also advise performing general organ health checks, most commonly through blood and urine tests and checking your cat’s blood pressure. This is because:

• symptoms of hyperthyroidism can mimic other common illnesses in senior cats, and
• senior cats may unfortunately have more than one issue at a time (such as hyperthyroidism alongside chronic kidney disease, heart disease and/or hypertension), which will affect their treatment recommendations

Treatment options for hyperthyroidism

Once your vet has confirmed a diagnosis of hyperthyroidism and screened your cat for any concurrent health conditions, they can then make recommendations as to appropriate treatment options. Your preferences and your cat’s temperament are also an important consideration here, as in practicality, not every option suits every pet!

Overall, the two main treatment options for hyperthyroidism in cats are:

1. Daily, long-term management with medication or a prescription diet: These therapies help to block or reduce your cat’s excessive thyroid hormone production, but don’t actually treat the underlying abnormal thyroid tissue
2. Permanent treatment/removal of their abnormal thyroid tissue: This is done with radioactive iodine therapy or surgery

Key points about long-term medications

• Commonly used options in Australia are carbimazole or methimazole – these are usually administered twice daily, either orally (as a tablet or liquid) or via the application of a (specially compounded) medicated gel absorbed through the skin on the ears
• As their abnormal thyroid tissue grows, cats may need increased medication doses over time to control their symptoms, so regular recheck blood tests are required
• Potential side effects include transient tummy upset symptoms, and much less commonly, itchy skin lesions, liver problems or bone marrow problems that cause reduced blood cell levels

Key points about a prescription thyroid diet

• These iodine-limited diets can work well in around 80% of cats, but require the cat to eat only this diet, with no other foods or treats – this can prove unfeasible in multi-cat households, or in cats with picky appetites, or those requiring prescription diets for other medical issues

Key points about radioactive iodine therapy

• This is considered the ‘gold standard’ treatment for hyperthyroidism, as it uses the administration of a single dose of radioactive iodine to selectively target and destroy the abnormal thyroid tissue, curing the condition without a requirement for anaesthesia or surgery
• The treatment requires cats to be isolated in a special hospital area for around a week while they are still slightly radioactive, so is not suitable for some cats with concurrent medical conditions
• Potential risks include an inadequate response in around 5% of cats (requiring repeat treatment), and a small percentage developing temporary or permanent hypothyroidism (insufficient thyroid hormone), requiring daily treatment with a supplement in the short- or long-term

Key points about surgical removal of the thyroid glands

• If all the abnormal thyroid tissue can be completely removed, surgery can lead to a permanent cure
• Potential risks include anaesthesia risks (which can be significant in older cats with concurrent health problems), the risk of inadvertent damage to the nearby parathyroid glands (which can lead to long-term problems with the cat’s internal calcium balance), and the risk of abnormal thyroid tissue growing in locations that can’t be surgically reached

The great news is that with appropriate management/treatment and monitoring, most cats with hyperthyroidism can get back to feeling like their normal selves again, with purr-rific quality of life!

Supporting your cat’s health

The best way to monitor your cat’s health is through regular proactive veterinary health assessments. It’s recommended to book your pet for a general health check:

• At least every 12 months for healthy cats up to 10 years of age,
• At least every 6 months for cats 10 years of age and older

If your cat is being managed for hyperthyroidism or another chronic health problem, they should ideally receive a check-up every 3-6 months or anytime they show a change in their condition.

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1 Oct 2026