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What is hyperthyroidism in cats?

Feline hyperthyroidism is an extremely common condition that occurs when the thyroid gland in the neck becomes overactive. The thyroid glands produce a hormone (thyroxine) that 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.

Growth in the thyroid gland can cause hyperthyroidism, producing more active cells. However, these cells can develop out of control and instead produce thyroxine continually instead of the amount that is needed. High levels of thyroxine in cats can force the body into overdrive. The body is kept constantly at an unnaturally high level of energy, which places enormous strain on other systems, including the heart.

Left untreated, cat hyperthyroidism can be fatal and early diagnosis is key to ensuring other organs are not affected. Fortunately, there are several treatment options available that can help to manage an overactive thyroid in cats effectively.


Symptoms of hyperthyroidism in cats

Some cats may show no visible hyperthyroidism symptoms, which can make it difficult to diagnose in the first instance. Some of the most common signs of hyperthyroidism 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, 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 can also identify the signs. 

Other important tests your vet may perform at the initial consultation include recording your cat’s weight and heart rate, conducting a urine analysis and taking blood pressure. These factors are all affected by hyperthyroidism and change according to the progression of the disease and the effectiveness of treatment. 

A few important things to note:

  • Other diseases can be confused with hyperthyroidism, so it’s important to confirm the diagnosis.
  • Some concurrent diseases can affect treatment and prognosis.
  • The degree to which the thyroid is producing excess hormone can affect the required dosage of medication. A blood test will give precise figures.
  • Some of the problems associated with hyperthyroidism, such as liver and kidney disease, 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 by an overactive thyroid.  

In hyperthyroidism, there are no physical signs, which makes it difficult to treat if symptoms are picked up too late. Therefore, it is best to start treating hyperthyroidism as soon as it is diagnosed.

Hyperthyroidism in cats treatment

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


There are several licensed medications available in the UK for hyperthyroidism in cats. If your pet is not responding well to their medical treatment, your veterinary surgeon may recommend trying a similar alternative. 

Medication should not be used with certain conditions such as diabetes and anaemia – another reason for running a blood test before treatment. In order to monitor effectiveness, as well as potential side effects, cats should have regular blood tests. Usually, we would begin testing at three, six, 10 and 20 weeks and then every three months following.

With hyperthyroidism in cats, medication will have to be given for the rest of the cat’s life, although it is not unusual for the dose to change over time. You should make sure to not split or crush the medication, and pregnant women should wear gloves when handling and when cleaning litter trays. 


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

Most often, your vet will recommend surgically removing just one gland, which will force the body to stop providing any thyroid hormone, which doesn’t cause your cat harm. If both thyroid glands are affected, still only one will be removed. This is mainly due to parathyroids – smaller glands attached to the thyroid glands – which control the level of calcium in the blood. When a thyroid gland is removed, the parathyroids might 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 there can be severe side effects, including seizures and paralysis, which can be fatal. This is why it’s 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 therapy is considered the safest and most effective treatment. The thyroid stores iodine, and small amounts of radioactivity can stop the overactive gland from producing too much thyroid hormone.

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 medication is not required long-term.

There are strict laws in the UK controlling radioactive substances, which means that a cat having radioactive iodine therapy will have to stay in a specialised isolation unit between five days and four weeks. 

Radioactive iodine is very safe and carries no significant risk or side effects. It is only offered at a few UK veterinary hospitals, so there may be some distance to travel for treatment.

Diet control for hyperthyroidism

Studies have shown that an iodine-restricted diet alone can control thyroid hormone levels, without the need for medication. Without iodine, the thyroid gland cannot make any hormones. Hill’s Pet Nutrition produces a prescription diet for cats with strictly controlled iodine levels. 

However, on this diet, your cat must not consume any other food or liquid apart from water. Obviously, this solution wouldn’t be practical for a cat that has access to the outdoors, or a cat that lives with others; even the smallest amount of stolen food will allow hormones to be produced. While there have been no reported side effects, it’s possible that very low iodine diets can affect the 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.


Side-effects of treatment for hyperthyroidism

As with any medication or treatment for pets, there can also be side effects. While 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 for cats can include:

  • Vomiting
  • Loss of appetite
  • Itchiness around the face
  • A lack of white blood cells, which can reduce immunity to other diseases


Side-effects of surgery, though rare, include problems with calcium levels. While this is curable, it may involve intensive treatment and hospitalisation. Calcium supplementation may have to be continued at home. Problems with infection or wound breakdown may also occur, although this is also very rare.


Treatment failures of hyperthyroidism in cats

More than 9 in 10 cats respond well to recommended treatment, although it won’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 hormone levels haven’t been stabilised. If your cat is not responding to treatment for hyperthyroidism, they will require extra monitoring from your vet.

The main reason surgery may not be successful is due to both glands being affected. In rare cases, there may be abnormal thyroid tissue inside the chest, and it’s not normally possible to diagnose this before both 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 the kidneys, which sadly is irreversible.

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


Complications of Hyperthyroidism in Cats

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 can lead to heart failure. The heart is a muscle, so when it is forced to work too hard for a long period, it gets bigger. Unfortunately, as the muscle grows, the space inside it for 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 continue to live a happy and healthy life.

Further Reading


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