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Hyperthyroidism in Cats

Advice about hyperthyroidism

Hyperthyroidism is a very common condition in older cats. It is caused by a benign tumour of the thyroid gland, which is responsible for normal metabolism. The tumour releases excess thyroid hormone which increases the rate of the metabolism of the body.

The increased thyroid hormones causes:

  • A very fast metabolic rate
  • Excessive nail growth
  • Unkempt hair coat
  • Thin body condition and loss of muscle
  • A very high heart rate which leads to heart disease and eventually heart failure
  • Change in behaviour

Signs you may notice:

  • Increased thirst
  • Increased activity/vocalisation
  • Weight loss – the most common clinical sign and often just put down to old age.
  • Increased appetite, cats appear very hungry and are always looking for food
  • Vomiting – can be occasional and sometimes only yellow bile or froth.


A clinical suspicion will be made after examining your cat and listening to their heart rate and sometimes we are able to feel the tumour in the consultation room. However we still always need to take a blood sample to confirm the increased levels of thyroid hormones in the blood, which can be taken and run at the clinic. We will also check kidney parameters because treating an overactive thyroid and therefore reducing the blood flow through the kidneys can unmask a kidney problem in rare cases.


There are 3 main treatment options for hyperthyroidism.

1. Tablets – These contain drugs that stop the excess production of thyroid hormones. This manages the condition but doesn’t cure it. It is a life long medication that is given in small tablet form twice daily. We will need to check thyroid blood levels and kidney levels every 3-6 months depending on how stable your cat is and make any necessary dose adjustments. The tablets are generally tolerated well, however initially in 20% of cases they will cause mild gastro-intestinal signs such as reduced appetite and vomiting which normally resolves. In 5% of cases they cause more severe signs e.g. severe facial itching and would then need to be stopped. We tend to start our cats on tablets initially to ensure we don’t unmask any kidney problems and to enable us to initiate treatment immediately. We can then either continue on this medication or move onto options 2 or 3.

2. Surgery – We can perform an operation at Bellevue Veterinary clinic to remove the thyroid tumour. It is successful in over 90% of cases and your cat will be cured. There is a possibility of recurrence if a tumour grows in the remaining thyroid gland however this is uncommon. This is a more expensive option initially but can work out cheaper in the long term. Side effects are present in less than 10% of cases although at their most serious can rarely cause calcium imbalance.

3. Radioactive iodine treatment - This is performed at our local veterinary referral centre in Langford. It is a very effective and safe procedure which requires just one injection of radioactive iodine under the skin which works to destroy all cancerous thyroid tissue. This will cure the condition and <5% of cats have side effects which are mild. However due to the fact your cat will be radioactive they need to stay in the hospital cattery for 10-14 days. For a further 2 weeks you will have to take further precautions at home e.g. they must stay in doors, and have a maximum of 10 minutes close contact with yourselves a day. If you are pregnant or have children below 12 then they need to stay at the centre for another 2 weeks. Before the radioactive iodine injection is given, your cat will undergo several tests to ensure they have no other serious pre-existing conditions including blood pressure measurements, chest x-rays, and a heart and abdominal ultrasound. This is the most expensive option but does rule out the need for surgery or ongoing tablets and blood samples.

These options will be discussed with you when the diagnosis has been made and a decision can be made by yourselves with your vet depending on the needs and health of your cat.

As with all our chronic conditions you will be entitled to a free nurse check every 3 months to keep an eye on your cat’s weight and general health.

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