Feline Hyperthyroidism

Feline Hyperthyroidism is one of the most common diseases seen in older cats at Castle Vets. It can occur in any breed or sex of cat, but usually occurs in cats over the age of 10 years old (although it is occasionally seen in younger individuals).

Hyperthyroidism in most cats is caused by a benign (non-cancerous) change in one or both of the cat’s thyroid glands, in rare cases it can also be caused by a malignant (cancerous) growth or change in the thyroid gland. Unfortunately no one knows what causes these changes to occur, but they both make the thyroid gland produce excessive amounts of the thyroid hormone Thyroxine.

1 shows normal thyroid gland.  2 & 3 show the parathyroid glands.  4 shows the abnormal thyroid gland

Thyroid hormones regulate many processes within a cat’s body including the metabolism, temperature, heart rate, blood pressure and bowel function. When too much thyroxine is produced the clinical signs can be quite severe, making the cat seriously ill. It tends to speed everything in the body up , which causes the cat’s energy to be used up very quickly.

Clinical Signs Of Hyperthyroidism

The symptoms of hyperthyroidism will vary from cat to cat. Some cats may have one or two of these symptoms while others may show several or all of the clinical signs.

  • Weight losshyperthyroid
  • Increased appetite
  • Increased thirst
  • Increased urination
  • Increased heart rate
  • Increased activity – restlessness and irritability (occasionally aggression)
  • Poor coat condition
  • Goitre – Being able to see or feel the thyroid glands in the cat’s neck
  • Vomiting
  • Diarrhoea
  • Heat intolerance
  • Panting
  • Occasionally a cat may have the opposite of the expected symptoms – e.g. loss of appetite, depression, weakness and lethargy.

If you notice that you cat is showing any of the above clinical signs then please contact your vet for an appointment as soon as possible. The quicker a health problem is diagnosed, the quicker your cat can get the treatment he or she needs to get back to their usual happy self.

Associated Complications Of Hyperthyroidism

Because of the changes it causes, hyperthyroidism can have serious associated consequences on other systems within the cat’s body such as

  • An increased heart rate and also changes (thickening) to the walls of the heart, which can then lead to heart failure if left untreated.
  • Hypertension (High blood pressure) is sometimes diagnosed at the same time as hyperthyroidism. If left untreated it can cause damage to the eyes, kidneys, heart and brain.
  • Kidney disease, while not directly caused by hyperthyroidism, is often seen at the same time because older cats are prone to both and treating them is often a balancing act between stabilisation of the two diseases. Some cats will show a deterioration in kidney function when they begin treatment for hyperthyroidism, this is not because the thyroid treatment is bad for the kidneys, but because in its early stages the hyperthyroidism causes an increased blood flow to the kidneys which can improve their function and help them work better; however it is actually masking the chronic damage and changes that the kidneys already have. When the hyperthyroidism is under control the kidney disease is made apparent.

How Hyperthyroidism Is Diagnosed

Your vet may use one or more of the following to diagnose your cat

  • Clinical signs and history
  • Thorough examination
  • Blood tests to check the levels of thyroxine in the blood stream and to see how well the other organs in the body are functioning.
  • Other clinical tests may be performed to help rule out or identify concurrent illnesses such as kidney disease or heart disease, such as an electrocardiogram (ECG), a blood pressure test or an ultrasound scan of the organs.

Treatment For Feline Hyperthyroidism

The good news is that there are treatments available for cats with hyperthyroidism and if the treatment is successful your cat should quickly return to his or her normal self which is fantastic for both the cat and the family.  The right treatment for each individual may vary and any other concurrent diseases or illnesses will need to be taken into consideration when deciding which one is best.

Tablets & Other Medications – Tablets that help to prevent the production of the thyroid hormones are usually given once or twice a day. They are simple to give and mean that the disease can be controlled without the need for an anaesthetic and surgery; however, they may suit all cats because a few may suffer side effects from the tablets such as inappetence, vomiting and lethargy. (Not to mention the cats who are very difficult to medicate!) These treatments are also available in a liquid form and as a gel that is applied to the ears (transdermal) twice daily. Care must be taken by owners to wear gloves when administering thyroid medication to their cats.

A blood test every 2-4 months is necessary to monitor the cat’s progress and ensure that the medication is effective and at the right dose.

tableting a cat

Tablets will not suit every cat

Prescription Diet Food – This is an option that has recently become available in the UK. The special diet, Hills y/d, has very strictly controlled levels of iodine in it. Iodine is used by the thyroid gland to make the thyroid hormones so the diet ensures that there is only enough iodine content to maintain normal thyroid hormone production. This works really well for some cats and is a great alternative if the cat cannot have medication or surgery, however, the cat must be fed on this diet exclusively with no other added foods, so it is often only a good option for an indoor cat and is unlikely to be beneficial in multi-cat households, unless strict monitoring and separation of the cats can be achieved at meal times.  A blood test every 2-3 months is necessary to monitor the cat’s progress and ensure that the food is keeping the disease controlled.

If kidney disease is present, then the vet may advise that your cat is given a prescription kidney diet instead of a thyroid diet.


Surgery – An operation to remove the overactive thyroid gland(s) is a frequently performed in many veterinary practices and the removal of the abnormal thyroid gland should completely cure the disease so there would be no need for further medication. The operation needs to be carried out by a skilled veterinary surgeon because if the parathyroid gland (which is very close to the thyroid gland) is damaged, it can upset the calcium levels in the body leading to further complications. Obviously the anaesthetic itself always carries a slight risk and this may be increased for older or unwell animals, so it is worth taking into consideration while discussing this as a possible treatment. the vast majority of cats recover quite quickly from thyroidectomy surgery and are home within a few days, but they will need to be closely monitored through examination and blood tests for the following few weeks before they can be given the all clear.


Radioactive Iodine Therapy – An injection of radioactive iodine is given to the cat, which destroys the abnormal thyroid tissues but does not affect the normal thyroid tissue. No anaesthetic is required and the majority of cats only need one injection in order to cure the disease. This treatment is, unfortunately, only available at a few veterinary specialist hospitals (Castle Vets is not one of these!) so there is usually a waiting list and the cost of the treatment can be quite high. Your cat will also have to stay at the practice with minimal handling, for around 4 to 6 weeks (until the radiation levels have dropped).


Contact Us

If you would like more information about Feline Hyperthyroidism or you are worried that your cat may be showing some of the associated clinical symptoms, you can contact us at the practice for more advice.

Tippy was diagnosed with diabetes last year

Tippy, who belongs to veterinary nurse Laura, has been diagnosed with Hyperthyroidism (and Diabetes!) and is doing really well on her thyroid tablets