Feline Hyperthyroidism

Feline Hyperthyroidism is one of the most common diseases seen in senior 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.

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. We do not know what causes these changes to occur, but 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 the body including 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 speeds everything in the body up , causing the body’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 – 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.

Other Complications Of Hyperthyroidism

Hyperthyroidism can have serious consequences on the cat’s heart. It causes an increased heart rate and also changes to the walls of the heart, which can lead to heart failure if left untreated.

High blood pressure (hypertension) 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 caused by hyperthyroidism, is often seen at the same time because they are both common diseases in older cats.

How The Vet Diagnoses Hyperthyroidism

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 X-ray.

Treatment For Feline Hyperthyroidism

The good news is that there is treatment available for cats with hyperthyroidism and if the treatment is successful the cat will quickly return to his or her normal self.

Medication/Tablets – 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!)

A blood test every 2-3 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 unlikely to be beneficial in multi-cat households.
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.


Surgery – An operation to remove the over-active thyroid gland (or both glands) 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. There is always a slight risk with any anaesthetic and surgical procedure and this may be increased for older or unwell animals. 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.
Most cat’s 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.


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 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 on 0118 9574488 or via the Castle Vets Website 

Useful Links

International Cat Care Hyperthyroidism 

MSD Hyperthyroidism Website 


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