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Cat health

Hyperthyroidism - over active thyroids

Hyperthyroidism is a relatively common disease of older cats (over 10 years of age). It is caused by an increase in the production of thyroid hormone from the thyroid glands, situated in the neck. Thyroid hormone has an important role in controlling the body’s metabolic rate. In hyperthyroidism, an excess of thyroid hormone results in an increase in the metabolic rate, so cats burn up energy rapidly and typically suffer weight loss despite an increased appetite. Fortunately, the vast majority of cats that develop hyperthyroidism can be treated very successfully and most cats will make a complete recovery.


The “classic” signs of hyperthyroidism include:

  • Weight Loss
  • Vomiting
  • Increased appetite
  • Diarrhoea
  • Rapid heart rate
  • Heart disease
  • Poor /unkempt coat
  • Restlessness

Clinical signs may initially be subtle at first, then more severe as the disease progresses. In some advanced cases, there will be generalised weakness, lethargy and loss of appetite. Cats with hyperthyroidism are also predisposed to the development of other serious and potentially fatal medical problems, such as hypertension (high blood pressure) and hypertrophic cardiomyopathy (heart disease characterised by thickening of the heart wall).


Physical examination may reveal the presence of an enlarged thyroid gland, fast heart rate or reduced body condition, suggestive of hyperthyroidism.  To confirm a diagnosis, a blood test is needed to measure the level of thyroid hormones in the blood. Usually, measuring the thyroxine (T4) concentration is all that is required to confirm a diagnosis, but sometimes additional tests may be needed (such as measurement of 'free T4' in the blood or scintigraphy). Other laboratory tests may also be abnormal. For example liver enzymes are commonly increased with hyperthyroidism, and both blood and urine tests are usually advised to help rule out concurrent problems (such as kidney disease). Blood pressure should also be checked where possible with hyperthyroid cats


There are four main options for treatment, each with advantages and disadvantages:

1. Medication

“Anti-thyroid” drugs, such as methimazole and carbimazole, are widely used in the management of both feline and human hyperthyroidism. These drugs reduce both the production and release of thyroid hormones from the thyroid gland. They do not provide a cure for, but they do allow either short-term or long-term control of hyperthyroidism.

They are available as oral tablets administered twice daily  or a once daily cat specific slow-release tablet. We can also source compounded flavoured pastes and liquids as well as transdermal gel preparation that can be applied to the inner ear.

The treatment dose is adjusted to effect and these drugs are usually both safe and effective. Side effects are uncommon. Poor appetite, vomiting and lethargy are most commonly seen, and these effects often resolve after the first few weeks of treatment. However, if gastric irritation persists alternative treatment may be necessary. More serious problems (bone marrow suppression, liver disorders, or skin irritation) are rarely seen, and if they occur require complete cessation of therapy.

Even if other surgery or radioactive iodine is considered (see below), drug therapy is often used initially to monitor the effect of successful treatment on kidney function.

2. Radioactive Iodine

Radioactive iodine is currently the GOLD STANDARD method of treating hyperthyroidism in both feline and human patients. This treatment involves the administration of radioactive iodine (I131) as a single oral capsule or subcutaneous injection. Radioactive iodine selectively irradiates all affected thyroid tissue, but spares adjacent normal tissues.

Radioactive iodine is effective in curing hyperthyroidism in over 95% of cases. In < 5% of cases, cats may remain hyperthyroid and require a second dose of radioactive iodine, or they may become hypothyroid (underactive thyroid hormone levels) and some may require thyroid hormone supplementation.

Due to radiation safety requirements, treatment can only be carried out at a licensed facility. We can refer cases to a few different centres across Melbourne. Your cat will need to be boarded at the veterinary hospital for 7 days following treatment, whilst radiation levels are highest. This compared to long term medication and routine monitoring, is often a very economical choice and is shown to increase a cat's lifespan compared to medications. In some patients with concurrent kidney disease, radioactive iodine therapy may not be suitable.

There is now a clinic and vet that deals solely with radioactive iodine. We would refer suitable cats through to them. You can find more details here and a very comprehensive run down of the disease and treatment. They are located in Box Hill

The Hyperthyroid Cat Clinic

3. Surgery

Surgical removal of the affected thyroid gland/s (thyroidectomy) can produce a permanent cure.  However, it is often not the first treatment recommendation due to the inherent risks associated with anaesthesia and surgery. In some cats, medical therapy is recommended prior to surgery to reduce hyperthyroid-related anaesthetic complications.

The major surgical risk is associated with inadvertant damage to the parathyroid glands, which lie very close to the thyroid glands and have an essential role in maintaining stable blood calcium levels. To minimise the risk of this complication, in cats that require surgical removal of both thyroid glands, it may be appropriate to perform the procedure in two stages. Even after successful surgery, signs of hyperthyroidism may recur at some time in the future due to increased activity of previously unaffected tissue.

4. Dietary treatment

A new option that has more recently become available for managing hyperthyroid cats is to feed them on a special diet that has strictly controlled levels of iodine. As iodine is used by the thyroid gland to make thyroid hormones, if there is only sufficient iodine in the diet to make normal levels of these hormones, this can help to control the disease.

It does require feeding a special therapeutic diet that is only available from your vet, and cats need to consume this diet exclusively to ensure effective control. Nevertheless, some cats can have their disease controlled in this way and it may be an alternative to other forms of therapy for a number of cats.

Further information: 

Vetbook Hyperthyroidism

iCatCare Hyperthyroidism

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