Feline Hyperthyroidism Explained

Feline Hyperthyroidism Explained

Feline thyroid disease is most commonly known as an overactive thyroid or hyperthyroidism.  In general, dogs get underactive thyroids (low/hypo) and cats get overactive thyroids (high/hyper).   Hyperthyroidism is usually seen in cats over 7 years old. 

Cats have two thyroid glands situated in the neck and their job is to produce thyroid hormones.  These hormones are responsible for regulating many body processes, including metabolism.  When a cat has an overactive thyroid, or hyperthyroidism, this essentially means that their metabolism is going too fast – they are burning too much energy. 

The signs that you will have probably seen in your cat include weight loss but also being ravenously hungry – so they can eat a lot while continuing to lose weight.  They can also become very hyperactive and restless, almost to the point of manic (sometimes yowling), especially around food.  Other signs can include poor coat condition, drinking more and urinating more, vomiting and diarrhoea.

So, what is happening in the thyroid glands to cause this?  The vast majority of cases are caused by a non-cancerous change which cause the glands to become bigger and then produce more thyroid hormone.  The underlying cause of this change is unknown.  Very rarely, a cancerous tumour (thyroid adenocarcinoma) can be the cause of the disease. 

Although most hyperthyroid cats show an increased appetite and restlessness, in some cases they may show the opposite signs and be weak and have a loss of appetite. 

If untreated, hyperthyroidism can cause an increase in heart rate as well as changes in the muscular wall of the heart that will eventually cause heart failure.  They may also have high blood pressure which can cause damage to other organs including the kidneys.  Along the way, your cat will have a stressful time of feeling constantly hyperactive and hungry, no matter how much they are fed.

 There is no known prevention for hyperthyroidism but early diagnosis and treatment can dramatically reduce the secondary effects such as kidney disease, high blood pressure and heart damage.   

The great news is that there are a number of treatment options available for hyperthyroidism and your cat can return to their normal selves and continue to have a long and happy life.  Once diagnosed, treatment of hyperthyroidism should continue for the remainder of your cat’s life. 



Treatment of Feline Hyperthyroidism

Treatment of Feline Hyperthyroidism

There are a number of treatment options available for Feline Hyperthyroidism depending on the level of your cat’s disease, your cat’s temperament and the costs involved. 

Successfully treated cats will usually have complete reversal of all the signs of hyperthyroidism. There are four main options for treatment, each with their own advantages and disadvantages:


The most commonly used and effective drugs used to treat hyperthyroid disease in cats are methimazole and carbimazole.  Both drugs stop the thyroid gland from producing too much thyroid hormone.  They do not provide a cure but the disease can be managed successfully. Medication has to be given on a daily basis, most often twice a day, for the rest of your cat’s life.

The treatment dose is adjusted to effect and these drugs are usually both safe and effective. Ongoing blood tests are required, especially early in the treatment process, to be able to achieve the most appropriate dose.

The advantage you will have at the start of treatment is that, if your cat is ravenously hungry (most cats with hyperthyroid disease are), they will happily eat a tablet in wet food without a second thought.  Once the disease is managed, they will hopefully be used to this and continue with it.  However, sometimes we have to put the tablet directly into our cat’s mouth and this can be tricky to master, especially if your cat doesn’t like to be held.  See my tricks on giving a tablet to a cat here.

Methimazole is also available as a gel formulation that is applied to your cat’s ear and rubbed in.  This is called a ‘transdermal’ medication.  As it is absorbed through the skin, it’s really important that you wear gloves when applying it, otherwise you will be absorbing it into your own body.  See my page on applying methimazole gel here.




Surgical removal of the affected thyroid tissues, known as a thyroidectomy, can produce a permanent cure and is a common treatment for many hyperthyroid cats in some areas of the world.

However, surgery does not come without risks.  Your cat is likely to be older already which immediately increases the risk of going under a general anaesthetic.  There is also the risk of surgical complications with the major risk being inadvertent damage to the parathyroid glands.  These are small glands that lie close to, or within, the thyroid glands themselves.  Because of this risk, it’s important to ensure a vet with experience in thyroid surgery is chosen to perform the procedure.   Your cat is likely to remain in hospital for a few days after the procedure so that they can be monitored. 

It’s important to note that, even with surgery, it is possible for a cat to develop hyperthyroid disease again at a later date if previously unaffected thyroid tissue becomes diseased. 


Radioactive iodine therapy

Radioactive iodine (I-131) is a very safe and effective treatment for hyperthyroidism. It has the advantage of being curative in most cases with no ongoing treatment. The radioactive iodine is administered as a single injection. The iodine is taken up by the abnormal thyroid tissue, but not by any other tissues, resulting in a very safe treatment option. 

Unfortunately, this is the most expensive treatment option and is required to be administered by a specialist veterinary centre.  Depending on where you live, your cat may even need to be flown to the nearest treatment centre, adding to the cost.  Your cat will need to stay in hospital for a short period after the treatment. 

A single injection of radioactive iodine is curative in around 95 per cent of all hyperthyroid cats, but following treatment occasional blood tests are recommended to ensure normal thyroid hormone levels are being maintained.


Dietary treatment

Hills has provided another option for treating our hyperthyroid cats in the form of dry and wet food called Hills y/d.  This diet has controlled amounts of iodine.  As iodine is used by the thyroid gland to make thyroid hormones, if there is only very limited amounts of iodine in the food then this helps to  decrease thyroid hormone production. 

The advantage of this is that there are no risks or side effects, unless there is a dietary allergy.  The disadvantage is that your cat will not be allowed to eat anything else, ever.  This can be an issue in a multi-cat household or if you cat goes outside and is at risk of catching and eating prey, or hitting up the neighbours for food. 

Your cat will also require ongoing blood tests to monitor the effectiveness of the dietary treatment.



Treatment of thyroid adenocarcinoma

The rare cases of thyroid adenocarcinoma (cancerous tumour of the thyroid) are more difficult to treat so you should speak to your vet about what options are available to you. 




Monitoring the disease

Monitoring the disease

Having a hyperthyroid cat means that blood tests will likely become a routine part of your and your cat’s life (unless surgery or radioactive iodine therapy is chosen as a treatment option).  These will be more frequent early on in the disease process until your cat is deemed to be stable.  After this, tests are usually required every 6 months at least.  The blood and urine tests will also keep an eye out for any signs of kidney disease. 

It is a good idea, when your cat is diagnosed with hyperthyroid disease, to ask your vet for an approximate estimate for the regular tests so you can plan ahead.  However, these types of estimates can only be a ballpark as things can change in the future depending on your cat’s health.   

These check ups and tests are vital in monitoring the progression of thyroid disease as they give the vet an indication of how well the treatment is managing the disease.  This enables your vet to adapt the treatment plan so that your cat has the best chance of successful treatment and a good quality of life. 

When your vet sees your cat, they will also be looking at their weight to ensure it is increasing on the treatment plan.  You can also do this at home easily and it’s something I would encourage.  After spending a lot of money on diagnosis and medication, there is something rewarding about seeing real progress at home.  The good news is that you can easily weigh you cat with a set of bathroom scales at home – click here to see this video.