What is hyperthyroidism?

Hyperthyroidism is excessive production of the thyroid hormone in the body due to the presence of a benign tumour (95-98% of cases) in the thyroid gland. It is one of the most common hormonal disorders in cats. Hyperthyroidism may occur in dogs as well although rarely and is usually the result of a malignant tumour. The thyroid gland hormone called thyroxin is one of the most important hormones in the body. Its production in the thyroid gland is controlled by the pituitary gland in the brain. However, as a result of the tumour in the thyroid gland, the gland works overtime and does not respond to signals from the pituitary gland. Excessive amount of thyroxin in the body makes body work much harder than usual. Some sources claim that there is correlation between feeding wet food and hyperthyroidism in cats while others state that it occurs as a result of cats reaching old age. Genetic factors seem to be also involved.

Who gets it?

Cats older than 7 years of age are mostly affected however if can occur in cats as young as four. Breed does not seem to be an important factor and the reason why it happens is unclear.

How do I know that my pet suffers from hyperthyroidism?

The symptoms can vary significantly because the whole body is affected. The most common sign is good appetite with weight loss. Other symptoms that can be noticed are enlarged thyroid glands on palpation, abnormal heart rhythm called gallop rhythm, dry lustreless coat, aggression, chronic vomiting, diarrhoea and hyperactivity or lethargy. Increase in water intake and panting can also be noticed.

How is hyperthyroidism diagnosed in cats?

Having one or two of the abovementioned symptoms does not mean that your cat has hyperthyroidism. Blood tests are needed to confirm your vet’s provisional diagnosis. Total T4 is one of the blood tests often used to diagnose the disease. However it is not always conclusive because the levels of thyroxin in the body can be affected by concurrent disease and medications. Additional blood tests such as free T4 and T3 suppression tests may be needed to confirm the diagnosis. Your vet will advise you on the best tests available for your cat.

Why is treatment so important?
If the cat is left untreated it may develop multiple problems such as heart failure, increased blood pressure and blindness as a result of this. Chronic diarrhoea and vomiting can be often present and reduce your pets quality of life. Medications are readily available and cost effective and there are many treatments available.

How is hyperthyroidism treated?

Not all cats need the same treatment. The treatment depends of the health of the animal, risks involved, the owner’s ability to medicate the cat, and cost.

Neomercazole is a drug that is successfully used in the treatment of hyperthyroid cats. It comes in a form of tablet that is usually administered twice daily. It is the cheapest treatment option but sometimes it can be hard for the owner to medicate especially if the cat is not easy to handle.

Methimazole transdermal gel works like Neomercazole by suppressing the production of thyroxin in the thyroid gland. It is applied to skin instead of orally. It is a bit more expensive but easier to use, however THE OWNER MUST WEAR GLOVES OR APPLY THE GEL WITH A COTTON BUD because it can be harmful to humans.

Once your cat has undergone initial treatment with Neomercazole or Methimazole it needs a blood test two weeks after the treatment commenced. This is to confirm that the dose was high enough to be effective. In some cases a blood test to check kidneys and liver is done at the same time – your veterinarian will inform you on the tests that are needed. If the initial treatment was successful your cat will need six monthly check ups and blood tests to make sure everything is in order.

There is also a surgical option. Surgery consists of the removal of the abnormal thyroid gland tissue however the parathyroid glands can be damaged in this procedure and as a result of this the cat may have issues with low blood calcium. Also there is anaesthetic risk and the cat must be a good surgical candidate.

Radioactive Iodine Therapy is the latest treatment with a high success rate and can be done at specialised practices. It is successful in 96-98% of cases and provides a permanent cure. The drawback is the cost and the cat has to stay in hospital until all radioactive iodine is eliminated from the body.

Monitoring treatment is important

Once stabilised your cat will need a six monthly blood test to make sure the values of thyroxin are within target range. Also your veterinarian may check liver and kidney enzymes when needed since some cats may develop secondary problems (e.g. kidney and liver disease) and should be monitored closely. Talk to your vet about the best approach for your cat.

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