Macarthur Veterinary Group
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Hyperthyroidism in cats

What is hyperthyroidism?

Hyperthyroidism is a disease of cats that is caused by a mass in the thyroid gland (located in the neck), which causes an overproduction of thyroid hormones.

What do thyroid hormones do?

Thyroid hormones, T3 and T4, are involved in the regulation of many metabolic processes. Thyroid hormones have a role in virtually every organ system in the body. They play an important role in metabolism of carbohydrate, protein and fat, and in the regulation of the function of the heart. The thyroid gland requires iodine to produce T3 and T4, and we can use this requirement to our advantage during treatment.

What causes the disease?

Unfortunately nobody knows what causes tumours of the thyroid gland to develop. Interestingly, the disease was first named in 1979 and there is little evidence to suggest that it was around long before that date. Some suggest diet, lifestyle changes, pollution and pesticides are possible causes. Recent studies suggest that the disease could be immune mediated (caused by an overreaction of the body’s immune system), and there is also most likely a genetic component to the disease. However, these are all yet to be proven. The rise in numbers of diagnosed cases since 1970 could largely be due to increased awareness of the disease and its symptoms.

Does the tumour mean my cat has cancer?

Thyroid tumours that cause Hyperthyroidism are generally benign and are usually multinodular adenomatous goiter. Malignant tumours of the thyroid gland (carcinomas) do occur, but are non-functioning; therefore they don’t cause hyperthyroidism.

Which cats are more likely to get hyperthyroidism?

Hyperthyroidism is a disease of middle age to older cats (older than 6-8 years of age, average 11-12 years). No breed or sex is more or less likely to get the disease.

What are the symptoms?

As previously mentioned, thyroid hormones affect virtually every organ system; therefore symptoms can range widely! Some cats have signs suggesting dysfunction of several organ systems, while others show signs localised to one system.

The most common signs are:

  • Weight loss despite a normal or increased appetite
  • Drinking and urinating more than usual
  • Hyperactivity
  • Poor coat quality
  • Vomiting and diarrhoea.
  • Increased heart rate
  • Heart murmur or heart rhythm abnormalities (arrhythmias).

And less commonly:

  • Heat intolerance
  • Muscle weakness
  • Bulky stools (poos)
  • Panting

Early symptoms are often missed because the cats are usually still eating well, and the early signs can mimic those of other diseases.

Which other diseases cause these signs?

Renal failure (kidney disease) is also very common in older cats, and can cause cats to drink more, urinate more and lose weight. However, generally cats with kidney disease will have a reduced appetite. Many cats will also have both kidney disease and hyperthyroidism.

Diabetes will cause increased thirst, increased appetite and weight loss.

Various cancers can also cause signs of weight loss, vomiting and diarrhoea, and a multitude of other signs.

If your cat has a heart murmur or arrhythmia, we may also need to rule out heart disease

It is worth noting that some cats may have hyperthyroidism as well as other diseases, which can make diagnosis and management more complicated.


Hyperthyroid cats often have an enlarged thyroid gland, which we may identify after careful palpation. This will add to our suspicion of hyperthyroidism.

To confirm the diagnosis of the disease, blood must be taken for a full blood count and biochemical analysis and to measure the thyroid hormone (T4) level. Urine should also be tested to help rule out kidney disease and diabetes.


Once we have diagnosed hyperthyroidism, we must first decide whether or not to proceed with treatment, as there are some circumstances in which we may suggest not treating your cat.

There are three options for treatment of hyperthyroidism. Two of these are considered curative, while the third simply controls the disease at a manageable level. The treatment option that is right for your cat may depend upon whether he or she has any other diseases, such as renal or heart disease, the age of your cat, and finally, finances will also play an important role!

Radio-iodine therapy

The best treatment option available is radio-iodine therapy. This therapy can only be performed at a specialist centre and uses radioactive iodine to selectively kill the thyroid cells that are producing too much hormone. The benefit of this treatment is that it is usually curative, and leaves the healthy thyroid cells untouched, allowing the gland to function normally after treatment.

The cost of treatment depends largely on the health of the cat; cats that also have kidney or heart disease will require more careful monitoring and longer hospitalisation, which will increase the cost involved. We may recommend this for younger cats, where lifelong treatment with medications can be impractical.

Medical Treatment

If radioiodine therapy is not suitable for your cat (based on cost, age or other complicating diseases), anti-thyroid drug therapy can be started with a medication that comes in tablet form. This drug reduces the production of thyroid hormone and so slows the progression of the disease. This is a lifelong therapy, and requires regular blood tests to ensure the thyroid hormone is within the normal range (this is most important in the first 3 months of treatment and we recommend a blood test every 3 weeks during this period). Initially, we recommend tablets given 3 times a day, and reduce this to twice a day (this may vary depending on how well the thyroid hormone level is being managed).


Surgical removal of the thyroid gland is not as common in recent times due to the introduction of radio-iodine therapy, but is still practiced successfully in areas without access to this newer treatment. There are risks involved in the surgery, as the thyroid gland sits next to the parathyroid gland, which is responsible for controlling calcium levels in the body, and other blood vessels and nerves are also located nearby. There is also a risk involved in putting older cats under an anaesthetic. Some cats may also require thyroid replacement medication (Thyroxine) after surgical removal of the gland.

Complications of treatment

Renal failure and hyperthyroidism

This is a common occurrence. Interestingly, hyperthyroidism can often mask kidney disease, because with the increased metabolic rate, there is better blood flow to the kidneys and better kidney function. On treating the hyperthyroidism, kidney disease can actually worsen. Therefore, we may choose not to treat the hyperthyroidism at all (and focus simply on treating the kidney disease), or we may suggest a trial with medical treatment to see whether there is any impact upon the kidney disease. If kidney function remains good while on hyperthyroid treatment, we may be able to proceed with other treatment options.

Heart disease and hyperthyroidism

If your cat presents with a heart murmur as well as hyperthyroidism, we may recommend taking an ECG and/or chest x-rays to assess heart function and size. There are medications available to help manage the signs of heart disease in cats.

What will happen if I don’t treat my cat?

Weight loss and weakness caused by poor digestion capabilities and the increased metabolic rate will become more severe. Cats who go untreated will eventually develop heart failure and this is usually the cause of death.