Radioactive Iodine (I 131) for Cats

Radioactive Iodine (Radioiodine) Treatment for Cats with Hyperthyroidism

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 radioiodine dose is curative in around 95% of cats, up to 5% require a second treatment. 


What is hyperthyroidism?

Hyperthyroidism most commonly affects older cats (8+ years). It is usually caused by a benign (non-malignant) growth of the thyroid gland. This gland produces thyroid hormone and when the gland grows it produces too much.

What is thyroid hormone?

Thyroid hormone has many functions in the body. One of the most important functions is that it controls the basal metabolic rate - the speed at which cells burn energy in the body. When there is too much thyroid hormone, your cat's body goes into over-drive, causing changes in ageing organs like the heart and kidneys.

What are the symptoms?

  • Weight loss
  • Increased appetite (like your cat just can’t eat enough)
  • Occasionally a reduced appetite
  • Vomiting
  • Diarrhoea
  • Rapid heart rate
  • Matted, dry, dull fur
  • Increased thirst and toileting
  • Strange / unusual behaviour (crying, hyperactive, aggression)

How is hyperthyroidism diagnosed?

Hyperthyroidism is most commonly diagnosed through a blood test. Your veterinarian will often recommend additional blood, blood pressure and urine tests to rule out other significant disease (such as liver and kidney disease) that could impact your cat's prognosis.

What happens if hyperthyroidism isn't treated?

If left untreated, hyperthyroidism can cause hypertension (high blood pressure), heart disease, liver disease, kidney disease, blindness (due to retinal detachment) and can be fatal.

How can hyperthyroidism be treated?

There are three main treatment options:

  • Radioiodine Treatment
  • Medical treatment daily (for life)
  • Surgical removal of the abnormal gland
  • Special prescription diet-only feeding (indoor cats only)

Your cat has individual requirements so your vet will help you select the best treatment option for them.

Why treat with Radioiodine?

  • Older cats are more vulnerable to the risks of anaesthesia so your vet may recommend you avoid surgery.
  • Giving daily medications to cats can be difficult and stressful - missed doses may mean your cat is going untreated causing damage to other organs. Also ongoing blood tests to monitor the dosing is required as your cat's needs will often alter over the course of the disease.
  • The longer your cat has hyperthyroidism even with medical management the risk of it turning malignant is greater.
  • It is hard to control a cat's diet - any cat who goes outdoors can't be effectively treated by diet only.
  • Radioiodine treatment is a one-off injection, relatively painless and highly effective - around 95% of cats are cured after just 1 treatment. It is also very cost-effective.

How does Radioiodine work?

The vet injects a natural form of iodine that is radioactive (I131). The thyroid gland concentrates Radioiodine so the radioactive dose is concentrated in the problem gland where it destroys the overactive cells, bringing it back to normal function.

How safe is Radioiodine?

  • Radioiodine treatment is safe for your cat and your family.
  • Your cat will receive only a very low dose of radioactivity concentrated in the problem area. This is a well-used treatment and vets are confident there are no side effects or damage to other organs. Your cat's fur will not change or fall out and the cat does not feel sick with this low dose.
  • Even though it is low dose the vet will keep your cat in a specially set-up boarding facility for at least 7 days to be 100% safe.
  • After 3 weeks your cat has negligible radiation levels because the Radioiodine disappears very quickly.

What happens during treatment?

  1. Your cat will be admitted and carefully examined by a vet.
  2. A small patch of fur will be clipped over the shoulder for the iodine injection.
  3. Your cat will be assessed and cared for daily in a special boarding facility for at least 7 days.

Are there any risks to Radioiodine treatment?

  • 95% of cats respond to 1 dose of Radioiodine and have normal thyroid hormone levels 1 month later.
  • Up to 5% of cats treated need a second Radioiodine dose.
  • Rarely Radioiodine treatment can reduce the thyroid gland too much, meaning the cat doesn't produce enough thyroid hormone, in most cases this only lasts 4-6 weeks. This can happen in about 2% of cases. A very small number of these cases need ongoing hormone supplementation.
  • Hyperthyroidism can mask many changes occurring in your cat's body. Your veterinarian will do their best to assess this risk before treatment is given. Sometimes despite thorough assessment cats can exhibit kidney or heart disease following both the initial hyperthyroid condition and the Radioiodine Treatment.

Can hyperthyroidism ever recur?

This is rare. Studies have shown that, once cured, the disease will recur in only 2% of those cats during the following 6 year period.

What do I do pre-treatment?

  • Avoid fish in your cat's diet
  • Cats must stop all thyroid medication including YD food 10-14 days prior to treatment.

When can my cat come home?

Your cat can leave the boarding facility usually after 7 days. Some families prefer to board their cats for a full 3 weeks until the Radioiodine is out of their system (particularly if someone in the house is pregnant or young). If you want your cat to come home after the initial 7 days there are some important guidelines to follow to be careful:

  • Avoid close contact with your cat for more than a few minutes at a time (no sleeping on the bed for 2 weeks)
  • Don't let your cat lick you and wash your hands after stroking them
  • Take extra care when cleaning up your cat's toilet - wear gloves & wash the area thoroughly.

Your vet will give you full instructions at discharge time.

Harewood Veterinary Hospital Radioiodine Treatment Guarantee

A single individualised Radioiodine dose is curative in around 95% of cats, up to 5% require a second treatment. Harewood Veterinary Hospital offers discounted re-treatment for all cats treated with Radioiodine Treatment at our hospital. Conditions may apply.

50% deposit is required at the time of a confirmed booking. Please contact us for details and conditions.