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Hyperthyroidism is when the thyroid glands produce too much thyroid hormone. There are two thyroid glands which sit either side of the airway in the neck. Hyperthyroidism causes the glands to enlarge and sometimes can make your cat’s neck look swollen in extreme cases. But you’re more likely to notice changes to weight, appetite and behaviour first.
Thyroid hormones help control metabolic rate – in other words, how fast the engine of the body ‘ticks over’. As such, hyperthyroidism can cause symptoms in many different body systems, as well as giving rise to other conditions.
Hyperthyroidism is a very common condition in middle aged to older cats. The vast majority of hyperthyroid cases are caused by the growth of a benign tumour called an adenoma. These growths mean more thyroid hormone is produced, but there’s no invasion into the rest of the body. There are some very rare cases of adenocarcinoma which are malignant tumours (tumours that spread to other parts of the body) that can cause hyperthyroidism, but these are rare.
Hyperthyroidism was hardly ever seen until recent decades, so other factors such as diet and environment have been considered as causes for the overgrowth of the gland – but there’s no consistent evidence at this time to support this.
Early symptoms can be very subtle, so any changes seen in older cats are worth paying attention to.
The symptoms you may see at home are:
Your vet will also be looking out for:
Vet Dr Corinne Wigfall says “The symptoms of hyperthyroidism can be vague and similar to many other conditions we see in old age, like renal disease or diabetes. This is why a vet check and blood tests are so important to find the exact cause.
“As a vet I always recommend a yearly health check and blood tests in any patient over eight years old.”
Hyperthyroidism is always worth treating, especially if no other diseases are present. If the hormone levels are successfully returned to normal, cats can lead entirely fulfilling lives. Left untreated, cats lose weight so they become emaciated, and it’s very stressful to them to be permanently hungry.
From a medical perspective, untreated hyperthyroidism is associated with heart disease and blood clotting problems. This is because the persistently high heart rate changes the shape of the heart muscle. This in turn causes blood to clot within the heart. If one of these clots break away, we can see issues with seizures, breathing problems and hind-leg paralysis – all of which are very painful and often fatal.
If you have any concerns, don’t hesitate to speak to your vet. It’s worth treating early to give the best outcome for you and your cat.
If you think your cat has a problem with their thyroid function, speak to your vet. They’ll be able to perform a health check on your cat to see if they agree, or whether other causes might explain the symptoms. This may include:
If hyperthyroidism is suspected, the most common form of diagnosis is a blood test to measure high levels of thyroid hormones. These tests can be performed in the majority of first-opinion vets, and will usually give reliable results. If the results are borderline, your vet may elect to wait and re-test in a few weeks, or to treat and assess response to treatment.
In rare cases, they can refer your cat to a specialist referral centre to get a technetium scan – which uses imaging to look for enlargement. Your vet should also perform biochemistry tests to check your cat’s general health. This involves looking at kidney and liver function, and an echocardiogram for their heart. These results are important when thinking about which treatment option is best for your cat.
There are four main types of treatment for cats with hyperthyroidism. Your vet will be able to advise the best course of action for your feline friend based on your needs and the cat’s temperament.
Hyperthyroidism is a lifelong condition, so choosing the option that you can reliably do daily is essential, or to choose an option that will give a more permanent cure. Monitoring the response to treatment with regular blood tests is also an essential part of treating hyperthyroidism. You need to make sure the values are in a safe range – neither too high nor too low – and to ensure that the kidneys in particular are not suffering from the changes to blood pressure that treatment can cause.
Anti-thyroid drugs block production of thyroid hormone within the glands. These are most often taken twice daily, and are available in liquid or tablet form. A consideration of anti-thyroid medications is that they should not be handled by women who are pregnant or trying to conceive, as they have been associated with issues in developing embryos. But it is safe to have a cat on these medications in the same house as a pregnant woman.
Treatment with anti-thyroid medication can be adjusted depending on how effective it is. Your cat should experience a reversal of any hyperthyroid symptoms for as long as they stay on the medication. If you stop the medication, or give too low a dose, the symptoms will come back.
Many owners start with oral medications to stabilise their cat, before deciding if they want to consider options. But many are happy to remain on the medications, especially now liquids are available.
Surgery – called a thyroidectomy – involves removing the thyroid glands, usually just one at first. This can mean a permanent cure, or at least a long-term cure, as the tissue is no longer there to produce more thyroid hormone. It can re-develop later down the line if any remaining tissue becomes enlarged too, in which case a second surgery is needed.
Surgery requires a general anaesthetic, so ideally the thyroid hormone levels are stabilised first to make the anaesthetic safer. This should be considered for older cats or those with additional health needs.
There’s also a possibility of damaging the parathyroid gland, which is a tiny gland right next to the thyroid that’s involved in calcium metabolism. Any damage to the parathyroid might leave your cat needing lifelong treatment for low calcium levels, and this medication is expensive. Another risk with surgery is if thyroid levels drop too low, and supplementation with tablets is needed.
After surgery, the signs of hyperthyroidism should resolve over several weeks, and hopefully your cat will no longer need medication. There is a small risk that – if just one gland was removed – the other gland may show signs later down the line, and a second surgery or other treatment needed. Your cat still needs to have regular blood tests to make sure their thyroid hormones stay at normal levels.
Radioactive iodine therapy may sound daunting, but it’s a safe and effective treatment that can bring about a permanent cure. Thyroid hormone needs iodine to be produced, and cats only get iodine through their food.
In radioactive iodine therapy, a small dose of iodine is injected under the skin, which is absorbed directly by the thyroid without affecting other organs. This targets and permanently inactivates the overactive thyroid tissue, as it uses this iodine to make thyroid hormone.
Because of the radiation risk, this treatment is done in specialist facilities where they can safely remove the radioactive waste your cat produces over the coming days. There are many clinics that offer this in the UK, and the costs involved are always coming down, meaning that the total is often comparable to lifelong medications and blood tests.
Many cats experience a complete reversal of symptoms after one radioactive iodine injection, and can go on living their adventurous lives. Occasional blood tests may be recommended to check symptoms have not returned.
Another treatment for hyperthyroidism is a low iodine diet, which controls the amount of thyroid hormone able to be produced. This is usually available on prescription from your vet. It’s important that your cat eats this food and absolutely nothing else, so it might not be the best choice for hunters, or those who share food with other cats.
If your cat is fed an exclusively low iodine diet, then they shouldn’t show symptoms of hyperthyroidism. If you change their diet or if they eat other treats or food (including mice!), then symptoms will not be under control, and your cat will still suffer from the issues hyperthyroidism can cause.
If they are a hunter, or snack on anything other than their special diet, a low iodine diet may not be enough to control their thyroid levels.
This treatment is normally best suited to indoor cats. As with medications, regular blood tests are recommended to make sure the thyroid hormones are in the normal range.
“Hyperthyroidism is one of the conditions I see most commonly in middle-aged to older cats. It’s always something I’d consider as a cause for weight loss or changes in behaviour, eating or drinking habits.
“These days very few cats present as the textbook skinny angry cat, with most owners bringing their cats in for far more subtle symptoms. Your vet will always want to rule this disease out, and will usually recommend a simple blood test that can be done in the practice.
“Hyperthyroidism is always worth treating, as the issues seen can mostly be reversed by getting the hormone levels right again. It’s very rare to find a case where none of the available options are suitable, especially now liquid medication is available.
“You do need to keep an eye on kidney function in hyperthyroid cats, as the disease can mask the kidneys not doing so well. So don’t be surprised when your vet asks to check these as part of monitoring blood tests. Left untreated hyperthyroidism can lead to heart disease and blood clots that can be really nasty, so I’d always say to treat, as hyperthyroid cats can still lead full and healthy lives.”
Dr Siân Burwood – MA VetMB MRCVS
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If your cat follows a dietary therapy treatment plan, then your vet should be able to prescribe a specific type of cat food. This will help to stop your cat’s thyroid from becoming overactive.
A few treatments such as surgery or radioactive iodine therapy can offer a long-term cure from hyperthyroidism. No treatment has a 100% success rate, but many cats can live long, happy lives after these treatments.
One of the side effects of hyperthyroidism, if left untreated, is that it can cause something called hypertension or high blood pressure. This can directly affect your cat’s vision, including retinal detachment. But you can avoid that by spotting symptoms and getting treatment early.
*Cover for pre-existing medical conditions is subject to acceptance. They will not be covered unless you have declared them and they are shown on your Confirmation of Cover.
**Illnesses and symptoms won’t be covered in the first 14 days and accidents won’t be covered in the first 2 days of the policy. This is unless you are switching from another lifetime pet insurance product with no break in cover and the claim you want to make would have been covered by your previous policy. Other exclusions may apply to other sections.