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Hyperthyroidism in cats

Cats as they age commonly develop a disease which causes an increase in thyroid hormone called hyperthyroidism. The thyroid is the accelerator pedal of the body. Increased thyroid level causes problems throughout the body including the heart, kidneys, gut and brain.

Treating thyroid disease can be very rewarding. Many changes in your cat that have been put down as old age changes are often reversed once the thyroid levels normalise. When things go well they put on weight, have a thicker, shinier coat and become more happy and relaxed. 

Signs of hyperthyroidism

  • Change in personality. Cats become needy and insistent. They will often want to be fed constantly. Cats become more agitated.
  • Weight loss with increased appetite and water intake. Despite eating more they are losing weight. Often cats also vomit and have diarrhoea.
  • Rapid heart rate. A hyperthyroid cat will often have a heart rate over 200 beats per minute. (Normal is 160 bpm).

  Make an Appointment with the Vet

or call us on 9913 7979


Clinical signs and history can often alert us to possible thyroid disease but blood tests are needed for a diagnosis.

Thyroid disease can usually be diagnosed with a routine blood test. Our Full Profile measures T4 along with many other factors. Many of our pets have the full profile now as part of their regular health care. This has meant we have excellent awareness of thyroid levels and changes that could cause your cats severe disease. 


  • Daily medication

The most common treatment is giving tablets or intradermal creams every day to control the thyroid level. These are usually given twice daily and their effect on the body needs to be regularly monitored with follow up blood tests. If the medication is not given the thyroid levels will rise again and start to make your cat unwell. 

Monitoring bloods and physical examinations are very important as thyroid disease is often present with other diseases such as kidney disease and high blood pressure. Careful dose adjustment is needed for the best results.

  • Radioactive Iodine Treatment

This specialist treatment involves giving your cat a low dose of radioactive iodine which concentrates in the thyroid tissue, killing the hyperactive thyroid cells.  Cats are kept in hospital for a number of days when the radiation levels are highest. When sent home, you must avoid being in close contact with your cat for a few weeks as the low level of radiation gradually decreases. This treatment regime is all clearly explained to you before the cat is treated. 

Radioactive Iodine Treatment (RIT) is a permanent treatment and is effective in 95% cases. The cost of this treatment has actually been decreasing and is now around $2000. It is really worthwhile to consider this treatment especially with the inconvenience of twice daily medication and the cost of on-going life long medications. 

Monitoring blood tests are still needed after RIT and a yearly Full Profile is still recommended. As with all elderly cats regular urine tests are also very helpful.

  • Dietary treatment

Some cats have had their hyperthyroidism controlled with dietary treatment. These special veterinary diets have extremely low levels of iodine. The aim is to make it difficult to produce thyroid hormone. Most foods contain iodine so the diet needs to be fed as the entire ration. If any other foods are eaten the diet will not work.


Thyroid disease is very common in cats over 10 years. If left untreated the high levels of thyroid hormone will cause problems in all the cat’s systems. Those changes will be ultimately fatal.

By treating your cat with medication you can appreciate the health improvements of controlling the thyroid disease. Ultimately, specialist referral for radioactive iodine treatment will give you the best long term result for your cat. It is a permanent treatment and relieves the burden of having to constantly medicate your pet.

  Make an Appointment with the Vet

or call us on 9913 7979

  Request a medication refill


Hypothyroidism in dogs

Dogs less commonly have thyroid disease and generally have a problem with low thyroid levels. We measure T4 in our Full Blood Profile and one low reading is often not significant.

Signs of hypothyroidism include hair loss and dull coat, decreased energy levels, weight gain and a slow heart beat. 

If your dog has a low T4 along with some of the signs of hypothyroidism we may start them on medication to supplement their thyroid hormone. If this is helpful, you will notice positive changes in your pet within a month. 

Dogs on thyroid supplementation will need a yearly medication review to monitor thyroid levels and all the other comprehensive indices. 

Neville’s hypothyroid problem.


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