Iodine 131 Treatment

Radioactive iodine (I 131) can be used to provide a safe and effective cure for hyperthyroidism, which is a common disorder of older cats.  The radioactive iodine is taken up by active thyroid tissue, but not by other body tissues, selectively destroying the affected thyroid tissue. A single injection of I 131 is curative in around 95% of cases, and in those cats where hyperthyroidism persists, treatment can be repeated.

A single treatment of iodine 131 is often curative and has no serious side effects.  Given by injection, the treatment is painless and does not require an anaesthetic. Precautionary measures are required to protect our staff during the treatment period.  For this reason the treatment can only be carried out at an approved facility, and the cat must be kept in the licensed hospital until the radiation levels have fallen to within acceptable limits.  This means cats must be kept in hospital for 5-7 days following treatment. Spacious cattery style accommodation is provided in our radioactive iodine ward. Ourvets Riccarton/ Ilam is an approved and licensed facility for this form of treatment.

Our isolation suits are similar to the boarding facilities – spacious single story rooms with glass doors. Our isolation unit only holds 4 cats, so we can deliver a gold-standard treatment for your pets.

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Client information

Please read our FAQ below and feel free to contact us if you have any further questions.

If your cat is going to be treated with radioactive iodine (I 131), it is very important to read our Client Discharge Information Sheet before collecting your cat from the cilnic and follow the recommendations.


What is hyperthyroidism? 

Hyperthyroidism (or an overactive thyroid) is a very common hormonal disease seen in older cats. The underlying cause is a tumour of the thyroid gland. In the vast majority of cases the tumour is benign. In rare cases (less than 3% of cats) it is caused by a malignant tumour. The thyroid tumour produces an unusually high level of thyroid hormone. Cats with hyperthyroidism commonly lose weight and become very hungry and thirsty. Other signs include vomiting, diarrhoea and agitation.

Hyperthyroidism is diagnosed by blood tests.

How is hyperthyroidism treated?

There are four ways that hyperthyroidism can be managed, each with associated pros and cons.

  • Medical treatment with daily pill or gel onto ear. This is a lifelong treatment. The dose of medication needs to be adjusted based on the results of blood tests.
  • Dietary therapy with iodine restricted prescription diet. This option can be successful for indoor cats but can be more difficult if there is more than one cat in the household or if your cat goes outside and might have access to other foods.
  • Surgery to remove affected thyroid gland(s). This is now a less common treatment in New Zealand due to the availability of radioactive iodine treatment.
  • Radioactive iodine treatment.

Your vet can discuss the advantages and disadvantages of different treatments with you. Each cat needs to be assessed as an individual and a tailored treatment regime discussed.

What is radioactive iodine treatment?

A single dose of radioactive iodine is injected under the skin. A light sedation may be given before the injection. When a radioactive form of iodine is given, the radiation destroys a portion of the thyroid gland. In most cats, only enough radioiodine is absorbed to destroy that part of the gland that is functioning abnormally.

This treatment is curative in approximately 95% of cases. A small number of cats will need a second injection of radioactive iodine. There is a very small risk of your cat becoming hypothyroid (having an under-active thyroid). This is very unlikely with the doses used at Ourvets.

A follow-up blood tests is recommended around 6 weeks after your cat is discharged from the hospital to see if the thyroid hormone level has come back down to a normal level. This can be done with your normal vet.

How do I arrange radioactive iodine treatment?

You need to initially discuss this with your own local vet (if you are not registered with Ourvets) who will then be able to refer you to our Riccarton/Ilam clinic for treatment.

How much will it cost?

The cost of treatment varies on the dose of radioactive iodine given to your cat. We carefully calculate this based on your cats thyroid hormone levels, clinical signs and size of the enlarged thyroid gland.

Please contact our friendly team at Ourvets Riccarton directly for current cost of treatment.

What happens when my cat comes in for treatment?

We ask that you bring your cat to our Riccarton/Ilam clinic before 10am on the day of the radioactive iodine injection. All thyroid medication (or iodine restricted diets) needs to be stopped 14 days before this (unless otherwise discussed with the vet.) The radioactive iodine injection is given around midday, usually under a light sedation. Following this your cat is hospitalised in a special radioiodine ward. The ward is similar to a cattery, with large pens, containing shelves at different levels.

You are welcome to provide bedding and toys for your cats stay if you feel this will help them to settle in. Please be aware that we have to dispose of these items once your cat leaves (due to the risk of contamination with radioactive material.) If your cat has a particular diet or special food that they like you are more than welcome to bring that with you.

Can I visit my cat?

Unfortunately this is not possible due to health and safety regulations. We are however happy to provide updates on a regular basis. You are welcome to call us at any time to see how your cat is doing. We are also happy to provide updates by phone or text message. Most cats settle in very quickly. Once the injection has been given the period of hospitalisation is no different from a normal cattery stay.

How long does my cat need to stay for?

Cats can leave our hospital after 5 days or 7 days (depending on the dose of iodine administered), provided the following restrictions can be observed:

  • Avoid prolonged periods of direct contact with your cat for approximately two extra weeks (e.g. keep cats out of bedrooms at night.)
  • Handle waste (urine, faeces, vomit) with rubber gloves and double bag any waste before disposing of in general rubbish for approximately two weeks.
  • We provide you with a full set of discharge instructions detailing recommendations for your cat’s care after iodine 131 treatment. We give you a copy when your cat is admitted and when they are discharged.
  • We offer hospitalisation in our iodine ward for two weeks after treatment for an extra charge if you have any concerns about being able to comply with these instructions.

If your cat is going to be treated with radioactive iodine (I 131), it is very important to read our Client Discharge Information Sheet before collecting your cat from the cilnic and follow the recommendations.