Hyperthyroidism In Cats

What is Hyperthyroidism? 

Feline hyperthyroidism is a common disease that affects cats. Hyperthyroidism enlarges the thyroid gland in a cat’s neck, due to the overproduction of thyroid hormones. In most cases, the swelling is harmless but in around 1-3% of cases it can be caused by malignant thyroid gland tumours – a specific type of cancer known as thyroid adenocarcinomas. 

Common symptoms of hyperthyroidism

Common symptoms of hyperthyroidism in cats include:

  • Weight loss
  • Increased appetite 
  • Sickness & Diarrhoea 
  • Increased thirst
  • Excessive urination
  • Low energy
  • Unkempt look to fur

Some less common symptoms include hyperactivity and some cats can become aggressive due to the distress of the condition. In some cases you may be able to feel small lumps under the skin on your cat’s neck, depending on the level of swelling. Some cats also experience hair loss and their heartbeat may increase. 

What causes hyperthyroidism in cats?

Feline hyperthyroidism is caused by the overproduction of thyroid hormones known as T3 and T4, which causes swelling in the thyroid glands. In the majority of cases, the swelling is the result of a benign change to one or more of the thyroid glands called ‘hyperplasia’ or ‘adenoma’. Whilst lots of studies have been conducted, there’s no single cause that’s been outlined for hyperthyroidism, however, according to research it’s more common in cats that live predominantly or permanently indoors and consume tinned cat food. Studies have also suggested that chemicals such as fire retardants could cause hyperthyroidism although this has never been confirmed. In rare cases (around 1 – 3%) hyperthyroidism is caused by a unique style of cancer known as thyroid adenocarcinomas.

Is hyperthyroidism painful?

The thyroid gland is an extremely important body part as the hormones it produces can effect almost every organ in the body. It can make the heartbeat faster, causing distress and risking cardiac arrest if it’s not treated swiftly. It can also cause hypertension or high blood pressure which in result can lead to issues with your cat’s eyes, brain or kidneys. Cats with untreated hyperthyroidism can experience pain and a low quality of life, so early diagnosis and treatment is key. 

How common is hyperthyroidism in cats?

Feline Hyperthyroidism affects around 1 in 10 cats over the age of 10 years old. With 10% of senior cats experiencing hyperthyroidism in some form, it’s a very common disease. The positive from this is it’s easy for vets to diagnose and there are lots of treatments available. 

What age do cats tend to get hyperthyroidism?

The majority of cases of feline hyperthyroidism occur in cats aged 10 years old or older. Only 5% of total cases are seen in cats under the age of 10, and it’s very rare for a kitten or young cat to be diagnosed with hyperthyroidism. 

How to prevent hyperthyroidism

There are no known preventative measures for hyperthyroidism at this time, as it’s not known what causes it. The best way to reduce the risk of your cat getting sick is to ensure it’s up to date on vaccinations and has regular checks at the vets. Pay attention to any changes in behaviour such as loss of appetite, increased thirst or unkempt looking fur. If any of these changes occur you should see your vet immediately and have tests done to diagnose your pet and allow them to get the correct treatment. 

Stages of hyperthyroidism in cats

Cats are very good at masking the signs of illness, as they tend to hide away if they’re not feeling well. Some of the first signs to look out for are weight loss which may not be noticeable day-to-day, but become apparent during checkups at the vets. Cats with hyperthyroidism also over eat, which can be mistaken for a healthy appetite or being in need of worming treatment. In the first stages the symptoms usually seem unimportant and can be mistaken for a lot of different illnesses known to affect older cats. 

If left untreated hyperthyroidism can advance, in the later stages your cat is likely to experience severe weight loss despite likely over eating, as well as becoming hyperactive. Your cat may also experience secondary health issues caused by the untreated hyperthyroidism such as issues with its heart which may lead to heart failure. Due to high blood pressure they may also end up damaging their eyes, kidneys or brain. 

How long can cats live with hyperthyroidism?

Untreated hyperthyroidism is almost always fatal which is why it’s so important to get your cat to a vet for diagnosis as soon as possible. With treatment and ongoing medication, cats can go on to live a long, happy life without any such reduction of their life expectancy.

How is Feline Hyperthyroidism diagnosed?

The diagnosis for hyperthyroidism consists of a vet doing a physical check of your cat’s neck to feel for any lumps and bumps before conducting blood tests to check for high thyroid hormone levels. If high thyroid hormone levels are found, the vet will likely produce a diagnosis of feline hyperthyroidism and discuss potential treatments with you. If your cat struggles with other illnesses then the diagnosis can be more difficult, as other illnesses may cause the thyroid levels to drop causing the vet to be unsure of the diagnosis. Further blood testing and imaging such as ultrasound tests of scintigraphy can be conducted to confirm the diagnosis of hyperthyroidism. 

Can cats recover from hyperthyroidism?

Yes – with the proper treatment cats with hyperthyroidism can go on to live a happy, healthy life with no reduction of their life expectancy. Total recovery can be achieved through surgery, or ongoing treatment through the use of medication. 

How to treat hyperthyroidism in cats

Treatment for hyperthyroidism isn’t a one size fits all solution, so upon diagnosis your vet will discuss the best options with you on a case by case basis. The most common solutions are:

  • Medication to slow down the overactive thyroid glands
  • Surgical thyroidectomy to remove the affected thyroid gland(s)


In most cases, the first course of action is to start your cat on antithyroid medication to reduce the overproduction of the thyroid hormone. The best medication for hyperthyroidism are medicines containing thiamazole, such as Thyronorm. Thyronorm is used for stabilisation of hyperthyroidism in cats prior to surgical thyroidectomy as well as for the long term treatment of feline hyperthyroidism. It takes several weeks for the medication to restore thyroid hormone levels to normal, but can be used on an ongoing basis to treat hyperthyroidism and cure the symptoms your cat is experiencing. Treatment is for life, if you stop medication, symptoms will return.


Another option for hyperthyroidism is to undergo surgical thyroidectomy, whereby the thyroid gland(s) are removed. Whilst this option may be expensive it will remove the need for ongoing treatment through medicines. Surgery isn’t an option for all cats, depending on their age and any other health complications/illnesses they may not be in a position to undergo the treatment. All surgeries come with potential complications and with surgical thyroidectomy there’s a tiny gland called the parathyroid that sits right next to each thyroid gland and controls your cat’s calcium levels. If the parathyroid gland is damaged during surgery it can lead to low calcium levels which will lead to your cat becoming poorly once again. The risks of surgery are greater if both thyroid glands are infected and need removing.

The cost of treating hyperthyroidism

If you decide to treat your cat with medication you will need to book in for regular check-ups with your vet and blood tests to ensure the medication is working correctly. Read the instructions on the medication carefully and ensure you administer the correct amount of medication. You can save money by requesting a prescription from your vet and purchasing the medication from a veterinary pharmacy such as The PharmPet Co, whereby you’ll save up to 70% of the cost compared to purchasing the medication directly from your veterinary practice.

If your cat is eligible for surgical thyroidectomy the costs can vary depending on other factors involved in the surgery and recovery process. Your cat will still need an antithyroid medication for several weeks prior to surgery to reduce thyroid levels. The surgeon may then take blood tests prior to surgery to confirm kidney function. Your cat will need to be anaesthetised for the surgery to go ahead, all of which adds to the costs involved in the surgery. The cost for the surgical thyroidectomy on its own costs in the region of £900 – £1,200, but the overall costs of supportive care and medications will likely increase this. Your cat will need further blood test after the operation to ensure it has worked correctly and hasn’t caused any other health issues such as a calcium deficiency. 

Are there any side effects to treatments for Feline Hyperthyroidism?

The main side effect associated with treatment of feline hyperthyroidism is ‘unmasking’ of previously hidden kidney disease. Kidney disease in older cats can be masked by hyperthyroidism, and only upon treatment the symptoms and damage to the kidneys may become clear. In other cases like with any medication, your cat may have a reaction to the treatment causing mild side-effects such as vomiting and reduced appetite. Less frequent reactions to medication include skin problems and facial itching. 

In very rare cases, treatment can also be too effective leading your cat to face the opposite problem of hypothyroidism – low thyroid levels. This can lead to cats being quiet and subdued, lethargic and losing their appetite. However, with regular vet checkups during treatment this should be quickly picked up by your vet and the dosage changed to counteract the problem. 

The best diet for cats with hyperthyroidism

Whilst there is no special diet required to support the treatment of hyperthyroidism, a low iodine diet can help to treat hyperthyroidism. Reducing the amount of iodine in your cats diet helps to prevent the thyroid overproducing thyroid hormone. This should only be done after speaking to your vet as low iodine diets can result in only specific foods being given.