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Hyperthyroidism is a common disease of the older cat, and frequently goes unrecognised by owners until the advanced stages. However, the good news is that this disease can be successfully treated. The article below is an over-view of the condition.

All animals have two thyroid glands in the neck – one on either side of the windpipe. These produce thyroid hormone whose main function is to control the metabolic rate of the animal. As metabolism affects every organ in the body, if things go wrong the problems are pretty far reaching. Some animals (and humans) can get conditions where the glands don’t produce enough hormone – hypothyroidism. Cats tend to go the other way and suffer from increased thyroid hormone levels due to growth of benign tumours on one or both glands (Fig.1). These tumours tend to affect older cats – typically 12 years or older, although we do occasionally see them in cats as young as six.

What signs will I see?

As thyroid hormone effects so many organs in the body, the whole well-being of the cat changes as the disease progresses. Initially the most common sign is weight loss. This is normally accompanied by an increased appetite and sometimes excessive drinking. Vomiting and diarrhoea often occur as well. Some cats will show behavioural changes becoming unsettled, hyperactive and very vocal. Because life isn’t simple, a few cats will do exactly the opposite – eating less and becoming lethargic.

As the disease progresses changes occur in the heart leading to cardiac failure and difficulty breathing. Rises in blood pressure may also lead to retinal bleeds in the eye, which can cause temporary or permanent blindness (Fig.3)..

A veterinary examination of a hyperthyroid cat will normally reveal other signs such as a very fast heart rate (typically more than 200 beats per minute) and an enlarged thyroid gland or ‘goitre’ in the throat. At this stage a blood test should be performed to check the cat’s thyroid hormone levels and also to assess the health of the liver, kidneys and blood cells.

Treatment options

Although hyperthyroidism is seem in older animals, it is a very treatable disease. Without medication or surgery an affected cat deteriorates, becoming extremely thin and weak and will eventually develop heart failure. Although this can take several months the condition will eventually prove fatal.

There are three options for treatment. The most ‘high-tech’ method uses the fact that iodine becomes concentrated in the thyroid gland of animals. This means that if a dose of radioactive iodine is injected into a cat, it ends up in the thyroid tumour where the radiation kills off the tumour cells without harming the rest of the body. This technique is extremely effective and in fact very safe. The only problem is that the cat is radioactive for about 4 weeks after the injection! To protect humans and other animals, they must be kept in lead-lined kennels, cared for by suitably protected staff and any urine or faeces disposed of as radioactive waste. Not surprisingly there are only a few centres in the country that have the facilities for this treatment. The closest one to us is in Canterbury.

The two treatments that are available at Darwin Veterinary Centre are anti-thyroid tablets and surgery.

Anti-thyroid tablets will usually bring the thyroid hormone levels down to normal within 1 – 3 weeks. We will generally examine treated cats after three weeks to check for weight gain, a slowing of the heart rate and possibly a reduction in appetite. A blood test may be performed at this stage to confirm that thyroid levels have dropped. The tablets are then continued long term, normally at a lower dose.

Although uncommon, there can be side effects to the drugs – occasionally vomiting and anorexia can occur; more rarely there can be effects on the cat’s blood cells. The main disadvantage of this treatment is that tablets only control the disease rather than cure it, so if the cat stops taking them regularly, the problem will recur. The dose required can change over time so cats treated in this way need to have blood tests every 3 – 6 months to ensure that normal thyroid levels are maintained. As most of us find it difficult enough to worm a cat 4 times a year, giving a tablet once or twice a day for the rest of our pet’s life is not an attractive prospect. Therefore we only tend to recommend this treatment for cats with other medical problems such as heart or kidney disease, which could make an anaesthetic risky.

Surgery to remove the enlarged thyroid glands is a relatively simple procedure and the vast majority of cats take the anaesthetic very well. We prefer to treat cats with anti-thyroid tablets for 2 – 3 weeks before the operation to slow the heart rate and normalise the metabolism so reducing the anaesthetic risk. The main advantage of surgery is that the cats recover very quickly from the op and require no further tablets. The only complication is that if both thyroid glands have to be removed, blood calcium levels can drop 2 – 3 days after the surgery. Fortunately, this problem generally resolves within a few weeks or months, although if calcium supplements are not given serious problems can arise. Therefore if we remove both glands, we will normally keep the cat in for 3 days to monitor calcium levels. Low blood calcium can occur in about 5% of cats who undergo the procedure.


Although overactive thyroid glands are a very common condition of older cats, advancing years are not an excuse to leave a cat to waste away when there are such effective treatments available. Radioactive iodine is the safest of those treatments, although it is expensive and having to lock your cat away in Canterbury for 4 weeks puts off a lot of owners.

Surgery is our preferred treatment as is sorts out the problem for good, while long term tablets never seem to be as effective – possibly because cats don’t always get their pill. Consideration should also be given to the cost of the regular tablets and blood tests, which overtime frequently exceed the cost of surgery.