Park Vets 256 Cassiobury Drive Watford Herts WD17 3PA | FOLLOW US ON FACEBOOK

Feline Hyperthyroidism

Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats that is caused by the overproduction of hormones from the thyroid glands, which are found either side of the windpipe at the base of the neck. Thyroid hormones control your cat’s metabolism (or rate of bodily activity); in this disease the thyroid glands go into overdrive and produce too much hormone which means that your cat will burn up energy too rapidly. Untreated this disease can be fatal but fortunately in most cases this condition can now be successfully treated.


What causes the disease?

In the majority of cases (98{c30ddf26c8e8213cb48863ffab783535ab04d34a51b7feafd4298625679b40f0}) the increase in thyroid production is caused by a benign tumour of the thyroid gland, however what causes this to develop is still not clear. In rare cases (2{c30ddf26c8e8213cb48863ffab783535ab04d34a51b7feafd4298625679b40f0}) a malignant cancer known as a thyroid adenocarcinoma can also be the underlying cause, this is much more difficult to treat successfully.


What are the symptoms?

This disease is almost exclusively seen in middle to old aged cats and is rarely seen in cats less than seven years of age. Most common signs are:

  • Weight loss
  • Ravenous appetite

Affected cats may also have:

  • Increased urination and thirst
  • Increased restlessness and irritability
  • Rapid heart rate
  • Unkempt coat
  • Occasionally vomiting
  • Diarrhoea
  • Lying in cold places

In about 1 in 10 cases the symptoms are unusual and the opposite of what might be expected, such as depression, loss of appetite and physical weakness.


How can my vet diagnose hyperthyroidism?

Apart from recognising the above clinical symptoms, there are a number of other steps in making a diagnosis. The thyroid glands may feel lumpy or enlarged. Blood tests are usually taken to rule out other diseases of the liver or kidneys. Directly measuring levels of hormone in the blood may help to confirm the diagnosis but in some rare cases the hormone levels may be normal.

There are a number of potential reasons for this and usually a repeat test one month later will show elevated levels. If not additional tests may be required to confirm or rule out hyperthyroidism. The vet will also want to check your cat’s heart and may also check his/her eyes. An early diagnosis is important to prevent secondary complications and reverse the disease process.


Secondary complications

Thyroid hormones have effects on virtually all the organs of the body, and therefore it is not surprising that this disease can sometimes cause secondary problems that may need additional investigation and treatment.

The effect on the heart is to stimulate a faster heart rate and a stronger contraction of the heart muscle, over time the muscle of the heart enlarges and thickens leading to a condition called ‘hypertrophic cardiomyopathy’, if left untreated this will lead to heart failure. It means that in some cats with hyperthyroidism additional treatment may be required to control the secondary heart disease. However once the underlying hyperthyroidism has been controlled, the cardiac changes may often improve, or even resolve completely.

Hypertension (high blood pressure) is another potential complication of hyperthyroidism and can cause damage to several organs including the eyes, kidneys, heart and brain; if hypertension is diagnosed along with hyperthyroidism then drugs may be needed to control the blood pressure to reduce the risk of damaging other organs. As with heart disease, successful treatment of the hyperthyroidism may cause the high blood pressure to resolve and additional treatment may not be required permanently.

Kidney disease (chronic renal failure) does not occur as a direct result of hyperthyroidism, but the two diseases often occur together as both are common in older cats. Care is needed where both conditions are present, as hyperthyroidism tends to increase the blood supply to the kidneys, which may improve their function.

Thus blood results taken to assess kidney function in a hyperthyroid cat may show normal or only mild changes, but potentially more severe renal failure may be masked by the presence of hyperthyroidism. For this reason irrespective of what treatment is chosen for long-term management of the hyperthyroidism (see below), it is advisable to start on medical treatment (tablets) initially and to monitor the response with repeat blood and urine tests to look at thyroid and kidney function.

Just occasionally, successful treatment of the hyperthyroidism results in a dramatic decline in kidney function. If this is detected it may be necessary to reduce the dose of therapy so that the hyperthyroidism is not fully controlled but renal function is not too severely compromised.



Because very few cats with hyperthyroidism have cancerous growths of the thyroid gland, treatment is usually very successful. The goal is to reduce the amount of thyroid hormone in the cat’s blood back to normal. This can be achieved in 4 ways:


a)  Medical Treatment

There are drugs available which block production of hormones by the thyroid gland. The medication is given one to three times a day.



  • Simple and does not require an anaesthetic
  • Suitable for cats with severe kidney disease which might be made worse by the sudden drop in blood pressure which may occur after other types of treatment
  • No dietary restrictions for your cat



  • Does not tackle the underlying problem and so treatment must continue throughout your cat’s life
  • Difficulties in getting your cat to swallow tablets or liquid. Transdermal gels are available, but these require careful application and handling
  • In some cats there are side effects ranging from tiredness to anaemia, skin irritation and liver changes.
  • In the early stages your cat must be carefully monitored to make sure that the dose is right. This practice then advises 6 monthly thyroid hormone and kidney function blood tests to monitor therapy. The dosage of medication is likely to change over time as the tumour continues to grow
  • Women of child bearing potential need to take precautions (gloves) when handling the tablets or their cats litter tray


b)  Surgical Treatment

The abnormal tissue can be surgically removed.

  • Advantages
  • Treatment should permanently cure the disease so no need for further medication


  • Disadvantages
  • NOT suitable for all cats, such as those with severe kidney disease or the very elderly.
  • Your cat may need drug treatment for a few weeks beforehand to show that its kidneys will cope and to stabilise their condition before anaesthesia
  • Requires a general anaesthetic which is always a slight risk, but more so in ill and elderly animals
  • Possibility of damaging the parathyroid glands, which lie close to the thyroid and control the use of calcium in the body. For this reason often only one side is removed at a time. This makes it likely that a second operation would be required, typically within 18 months of the first
  • Thyroid tumours can sometimes occur in the chest, making it difficult to locate and/or remove, this would result in the cat remaining Hyperthyroid, despite both Thyroid glands being removed.
  • After surgery the cats should be carefully monitored for a couple of weeks to make sure there are no changes in blood calcium caused by parathyroid gland damage.


c) Radiation Therapy

An injection of radioactive iodine will destroy the abnormal thyroid tissue while leaving normal cells unaffected.

  • Advantages
  • No anaesthetic required and very few unwanted side-effects.
  • A single treatment will permanently cure the disease in 9 out of 10 cases and a second treatment will do the trick for the rest
  • Radiation will also work in the rarer cases in which the tumour is malignant or where a portion of the thyroid tissue has broken away from the main gland and is normally missed during surgery


  • Disadvantages
  • Availability: This is only available in a few referral centres
  • Your cat will have to stay in complete isolation until the radiation level has died down, usually around four weeks. Cats can be sent home after the treatment in as little as 4 days, but it is usually closer to 14days. Strict handling rules would also have to be applied once home, usually for an additional 14 days, including no access to children, women of child bearing potential or pregnant women. Special precautions would need to be taken with the cat’s urine and faeces, and they must be kept indoors. Home management is not always achievable, in these cases the cat would need to remain at the referral centre for longer
  • Because of the need for limited handling, this method can be unsuitable for cats needing treatments for other serious conditions
  • The cost of treatment and prolonged boarding can be high
  • A small proportion of cats can become Hypothyroid (low levels of hormone) after treatment. Often no symptoms present, but a small number may need treatment


d) Dietary management

A prescription diet with an extremely low Iodine content. Without Iodine the thyroid glands cannot produce the hormone and Thyroid tumours cannot over produce it.

  • Advantages
  • No medication or invasive surgical treatments are required
  • Your cat can be treated at home by a simple change of diet


  • Disadvantages
  • Your cat can only eat the diet and nothing else, at all, ever. For cats in multi-cat households or who go outside this can be a problem. Even a small amount of other food can provide enough Iodine for the Thyroid to over produce Thyroid hormone.
  • If your cat will not eat the diet, because they do not like it or become unwell, then it will not work
  • It can take several months to work, although many cases respond in a few weeks
  • The thyroid tumour is only prevented from producing hormone, it can continue to grow
  • Periodic blood sampling to check how well the condition is controlled is still advised..


What is best for my cat?

The vets at the Park Veterinary Centre will help you make the decision on the best method of treatment for your cat after careful discussion in a consultation. There are a number of things to consider, your cat’s age, the severity of the condition, the presence or absence of other diseases and the risks of complications, etc.

Cost may also be a factor as both surgery and radiation treatment can involve initial expense. However, medication may prove to be costly in the longer term, especially as treatment has to be given for several years and your cat will need to have regular six monthly blood tests.

These tests will assess that the correct amount of medication is being given, and also ensure that your cat remains healthy, as it gets older despite its underlying disease.