Hyperthyroid Disease in Cats




General Information

The thyroid gland is a small pair of glands located either side of the windpipe in the lower neck. The thyroid gland produces thyroid hormones which are responsible for regulating the body’s metabolic rate. Hyperthyroidism is the overproduction of thyroid hormones caused by an overactive thyroid. In most cases this is due to a benign (non cancerous) enlargement of the thyroid gland. The thyroid hormones are important to many bodily processes and the over production of this hormone can have serious effects on health and wellbeing.

Hyperthyroidism occurs most commonly in middle to old aged cats.

Clinical signs of hyperthyroidism

The signs are wide and variable. The most common signs include:

  • Increased hunger and thirst
  • Restlessness or agitation
  • Weight loss
  • Rapid heart rate
  • Rough hair coat
  • Vomiting or diarrheoa


Many of these older cats may also have other concurrent diseases, e.g. heart, liver or kidney disease. These diseases may have an impact on the way that the cat is treated.

The onset of symptoms is usually insidious and may mimic other diseases that are seen in older cats which owners may not recognise as thyroid disease until it is in quite an advanced stage.


The original symptoms usually include a weight loss in spite of a good appetite. As the disease progresses the cats tend to become more and more demanding of food. They may have temperament changes and become more nervy and jumpy. Sometimes they become very vocal and demanding of attention. Often their thirst will increase, they are prone to gastrointestinal upsets as they cannot digest their food properly and their coats become rough.

Cats with hyperthyroidism that are not treated may develop a heart condition called hypertrophic cardiomyopathy, in which the muscle of the heart becomes excessively thick. This can lead to heart failure and death.

Steps to diagnosis:

  • General Physical exam (including palpation of the lower neck)
  • General health blood tests (including liver and kidney enzymes)
  • Total T4 (check thyroid level)
  • Urine test

All these tests help to give a full picture of your cats health and allow the veterinarian to asses a treatment plan as each cat is individual.

Treatment options:

There are three treatment options for feline hyperthyroidism.


1. Medication

Drugs are available that reduce thyroid hormone production, they come in a tablet form or in a cream form (transdermal) that is rubbed on the inside of the cats ear. However, these do not cure the disease but help manage the condition. These drugs often need to be given twice a day for the life of the cat and can have some side effects. The tablet form is the least expensive option but may have more side effects than the transdermal. These medications should not be given for two weeks prior to radiation therapy.

The dose of the drug used is adjusted based on blood tests that check the level of your cats thyroid hormone whilst on treatment. The dose rate of the medication may change. Initially the blood tests are more frequent but once your cats level are stabilised the testing becomes less frequent.


2. Surgery

Surgical treatment involves removal of the affected thyroid gland(s). This can provide a cure for the disease. However, due to the age of most cats affected and the possibility of heart disease there is some anaesthetic risk involved.


Complications may arise when both glands need to be removed (70% of cases involve both glands). This occurs when the parathyroid glands are removed with the thyroids. This is a common complication resulting in problems with calcium regulation.


3. Nuclear Medicine and Radioactive Iodine

The third form of treatment uses nuclear medicine technology and is considered by most experts in this area to be the method of choice for these older cats. It provides a safe and relatively simple cure for hyperthyroidism.


The thyroid gland needs iodine to manufacture thyroid hormone. Advantage can be taken of this to inject a quite small dose of radioactive iodine into the cat. This iodine is then extracted from the blood by the tumour cells for them to make more thyroid hormone. The tumour cells are therefore directly exposed to the radiation and so die.


The surrounding normal thyroid cells don't tend to take up the radioactive iodine and are spared from the radiation and can subsequently function normally again.


The only real disadvantage it seems to this treatment is the cost and the fact that the cats are slightly radioactive for a few days after treatment and must be kept in strict isolation. After this period the cats can return to their home environment but we advise owners not to have prolonged close contact with the cats for another week or so and to discourage cats sleeping on the bed or sitting on someone's lap for a couple of hours watching a movie on TV in this period.

Radiation therapy


A consultation with a referral veterinarian is needed for radiation therapy. This is normally done at a clinic in Sydney.


Protocol for radioactive iodine treatment

A strict protocol is adhered to in all nuclear medicine cases to ensure the safety of your pet, your family and the radiation staff.

In many cases before you are referred for a radiation consult we will do a 2 week treatment trial with a thyroid drug as its effects wear off once the drug is stopped and this helps ensure that there is no underlying kidney disease lurking that may change our decision to recommend radiation treatment.



To reduce any anxiety and enable thyroid scanning, all cats are sedated prior to their scan. The drugs used for this purpose are selected to have minimal side effects.


Thyroid Scanning

Before treatment a small dose of a radioisotope Technetium 99m is given to each cat. Once this has had time to concentrate in the thyroid gland (about 20-30 minutes) a radiation emission scan is taken. The amount of isotope taken up by the gland is used to calculate the therapeutic dose of radioiodine.


Method of Treatment

The radioactive iodine is given as a single injection under the skin. This causes minimal discomfort to the cat during treatment and recovery.




After treatment cats are kept in an isolation ward until their radiation levels are below Government EPA standards. During their stay, they are regularly checked by an  in-hospital vet, who will call you daily with an update on your cat's progress. They are happy to board your cat for longer if you have particular concerns about exposure, e.g. if there are young children in the family or if someone is pregnant.



Cats are not discharged until their output of radiation is deemed safe, usually after 5-7 days. For your added safety we recommend not letting your cat sleep on the bed or sit on your lap for long periods for another week. Daily cleaning of litter trays is also important as urine and faeces are the main way in which radiation is excreted.



Your cat should be rechecked by your regular vet 7 days after discharge. This enables them to assess the cat’s progress. You should also take your cat to your vet 6 weeks after treatment for blood tests to ensure that the thyroid hormone level has reduced. You can usually expect your cat’s behaviour returning to normal a month or so after treatment. Although the international recurrence rate of hyperthyroidism in cats is reported to be about 10%, often times at the referral centres in Sydney only 0.3% have required subsequent treatment.



Cats treated in this fashion often have developed a normal thyroid function within one week of treatment and have largely recovered from their disease within 4 - 6 weeks from treatment.