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What is hyperthyroidism in cats?

Feline hyperthyroidism is an extremely common condition and occurs when the thyroid gland in the neck becomes overactive. The thyroid gland produces a hormone (thyroxine) which controls the animal’s metabolic rate – i.e how the body uses energy. The brain controls the level of thyroxine by telling the thyroid gland how much to make.

A growth in the thyroid gland can cause hyperthyroidism, producing more active cells. However, these cells can become out of control and instead produce thyroxine continually instead of the amount that is needed.

This results in your pet having too much thyroxine in the body, where high levels can make the body go into overdrive. Instead of a single hit, the body is kept constantly at this unnaturally high level of stimulation and energy, which places enormous strain on the body’s systems, including the heart. 

Left untreated, hyperthyroidism in cats can be fatal and early diagnosis is key to ensuring other organs are not affected. The good news is there are a range of treatments available which can help to manage the condition effectively.  


Symptoms of hyperthyroidism in cats

Symptoms of hyperthyroidism in cats can vary; some may show no visible signs at all, which can make it difficult to diagnose in the first instance. Some of the most common and obvious symptoms to watch out for are:

  • Increased appetite, coupled with weight loss
  • Excessive thirst 
  • Increased urination 
  • Decreased activity levels

There are also many internal side effects associated with hyperthyroidism, the most common including; 

  • Liver/kidney damage
  • Heart disease
  • High blood pressure
  • Blindness
  • Brain damage


How is hyperthyroidism diagnosed in cats?

Only a blood test will give a confirmed diagnosis of hyperthyroidism in cats, however during a physical examination, your veterinary surgeon will also identify the signs. 

Other important tests your vet may perform at the initial consultation is to record your pet’s weight and heart rate, do a urine analysis and also measure their blood pressure. These factors are all affected by hyperthyroidism and will change according to the progression of the disease and the effectiveness of any treatment. 

This is for several reasons:

  • Other diseases can be confused with hyperthyroidism, so it is important to confirm the diagnosis
  • There may be a concurrent disease which can affect treatment and prognosis
  • The degree to which the thyroid is producing excess hormone can affect the dosage of medication, and a blood test will give precise figures
  • Some of the problems associated with hyperthyroidism, such as liver and kidney disease, can affect treatment and prognosis and these can only be evaluated by a blood test 


Does hyperthyroidism in cats need to be treated?

Feline hyperthyroidism can be quite a misleading disease. Often the cat can seem well and happy in themselves, although it’s not possible to see the damage that is being done inside.  

In hyperthyroidism, there are no physical signs to look out for which makes it difficult to treat if the symptoms are picked up too late. Therefore, it is best to start treating hyperthyroidism as soon as it is diagnosed, as this ensures both the best welfare for your pet and the best prognosis for long-term health.


Treatment for hyperthyroidism in cats

There are various options for treating hyperthyroidism, and your veterinary surgeon will be happy to discuss the advantages and disadvantages of each for your cat.


There are several licensed medications available in the UK for hyperthyroidism in cats. Two drugs are different but closely related, so some cats will respond better to one than the other. If your pet is not responding well to their medical treatment, your veterinary surgeon may recommend trying the alternative. 

All medication is contraindicated (should not be used) in certain conditions such as diabetes and anaemia, which is another reason for running a blood test before starting treatment. In order to monitor the effectiveness of the treatment, as well as potential side-effects, it is recommended that cats have regular blood tests. Your veterinary surgeon will recommend when is best for your pet to have their blood checked. Usually, we would begin testing at three, six, 10 and 20 weeks and then every three months following.

Medication will have to be given for the rest of the cat’s life, although it is not unusual for the dose required to change with time. Pet owners should make sure to not split or crush the medication, and pregnant women should wear gloves when handling and also when cleaning litter trays. 


Surgery is the second most common treatment for feline hyperthyroidism and involves removing the thyroid gland. There are two thyroid glands, one located each side of the windpipe in the neck. It is common for only one side to be affected at first, although in many cases the second gland will also become affected. 

Most often, your veterinary surgeon will recommend surgically removing just one gland, which will force the body to stop providing any thyroid hormone. The lack of thyroid hormone doesn’t cause your cat any problems. If both thyroid glands are affected, still only one will be removed. This is mainly due to there being other smaller glands attached to the thyroid glands, called parathyroids. These glands control the level of calcium in the blood. When a thyroid gland is removed, the parathyroids are disturbed, and this may cause them to stop working, which can be permanent.

This isn’t a problem if it happens only on one side, but if it happens on both sides at the same time (e.g. if both thyroid glands are removed at the same time) then there can be severe side effects including seizures and paralysis, which can be fatal. Therefore, if both your cat’s thyroid glands are affected, it is most common to remove one gland at a time, separating the operations by at least six weeks to allow the parathyroid glands to settle down.

Radioactive iodine therapy for hyperthyroidism

Radioactive iodine treatment is considered the safest and most effective, using a technique to give your cat iodine which has been made radioactive. The thyroid stores iodine, and therefore, the small amounts of radioactivity damage the overactive thyroid gland and stop it from working.

The amounts of radioactivity are too small to affect the rest of your cat’s body, and it is eventually passed out of the body via urine. No general anaesthetic is needed, and after treatment, medication is not required for long-term.

There are strict laws in the UK controlling radioactive substances, which means that a cat having this treatment will have to stay in a specialised isolation unit in hospital for five days to four weeks (depending on the treatment required). 

Radioactive iodine is a very safe treatment and carries no significant risk or side-effects. In addition, it is also only offered at a few veterinary hospitals in the UK, so there may be some distance to travel to a referral vet.

Diet control for hyperthyroidism

As mentioned, iodine is associated with the thyroid gland, and studies have shown that feeding a iodine restricted diet alone can control hyperthyroidism without the need for medication. Without iodine, the thyroid gland cannot make any hormones. Hill’s Pet Nutrition produces a prescription diet for cats that has very strictly controlled levels of iodine. 

However, your cat must not eat any other food or liquid apart from water for it to be effective. With this, the solution wouldn’t be practical for a cat that has access to the outdoors, or a cat that lives with others inside the house. Even the smallest amount of stolen food will allow thyroid hormones to be produced. While there have been no side effects reported with the prescription diet, it is thought that very low iodine diets can interfere with a cats immune system, although, there is no evidence of this. 

The diet is successful in approximately 90% of cases. Usually, if it doesn’t work it is because the cat is getting food from elsewhere. It is not known why it doesn’t work in some rare cases.


Side-effects of treatment for hyperthyroidism

As with any medication or treatment management for pets, similar to when humans receive treatment when they’re unwell, there can also be side-effects. 

How well the treatment performs is dependent on how well your cat responds. While any side effects are temporary and can be treated fairly easily, it’s still important to be aware of them so you can provide the best care for your pet.

Side-effects of thyroid medications can include:

  • Vomiting
  • Loss of appetite
  • Itchiness around the face (which can cause your cat to scratch and hurt themselves)
  • A lack of white blood cells, which can cause reduced immunity to other diseases

Side-effects of surgery include problems with calcium levels. While this is curable, it may involve intensive treatment, as your cat will need to be hospitalised for a period of time. Calcium supplementation may have to be continued at home, via a tablet. In addition, problems with infection or wound breakdown at the surgical site may also occur, although this is also rare.


Treatment failures

More than 9 in 10 cats will respond well to the recommended treatment option, although it can’t always work well for everyone. 

If medication isn’t working for your cat, it may just mean trying the alternative tablet. Any of the other management options discussed can be explored, even if the hormone levels haven’t been stabilised. If your cat is not responding to treatment for hyperthyroidism, they will require extra monitoring and your veterinary surgeon will discuss any additional risks that may be involved.

The main reason the surgery may not be successful is due to both glands being affected. In rare cases, there may be abnormal thyroid tissue inside the chest. It is not normally possible to diagnose this before both thyroid glands have been removed. Abnormal thyroid tissue can be controlled by any of the other treatments quite successfully, or your cat can be referred for specialist surgery.

Radioactive iodine is successful in more than 95% of cases. It will also target any part of the body that is producing thyroid hormones and can be used after surgery if required for ectopic thyroid tissue. Very occasionally, a second treatment will be required to get the full effect.


Concurrent diseases of hyperthyroidism

The high blood pressure and overwork of the body that feline hyperthyroidism causes can also damage their kidneys, which sadly is irreversible. This is a result of the overdrive forcing the kidneys to work harder, compensating for the damage. 

Once the thyroid hormone levels become normal, the kidneys go back to working at their normal efficiency, and any damage only then becomes apparent. This means it is very common for kidney disease to be diagnosed after the hyperthyroidism is treated, regardless of what treatment is used — it is not a reaction to the chosen treatment. 



The longer hyperthyroidism is left untreated, the greater the risk of significant underlying kidney disease. Kidney disease can be managed medically, but cannot be cured, and it has a far more significant effect on your cat’s welfare and life expectancy than hyperthyroidism.

The heart is another victim of both high blood pressure and being driven to work too hard by hyperthyroidism. The heart is a muscle, so when it is forced to work too hard for a long period, the muscle gets bigger. Unfortunately, as the muscle gets bigger, the space inside it for the blood gets smaller, and the heart actually becomes less effective.

The changes are irreversible and mean that the heart cannot get as much blood and oxygen around the body as it should. Your cat may become very tired or lethargic, and have a greater risk of having a stroke or heart attack. Medication can help, but the prognosis is guarded for a cat with this type of heart disease.


If you suspect your cat is suffering from hyperthyroidism book a free consultation with your local Animal Trust surgery to have them examined as soon as possible. Early diagnosis of the condition leads to effective management and ensures that your cat can be continued to live a happy and healthy life. 

Further Reading


Coronavirus Measures in place at Animal Trust

We have put in some new measures for all clients when attending appointments at one of our surgeries. Read more here.

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