Radioactive Iodine

Is Radioactive Iodine (I-131) right for my cat and me?

What is hyperthyroidism?

Hyperthyroidism is a common endocrine disease of cats. It is generally a disease of older cats with an average age around 9-10+ years, but can be seen in cats as young as 5 years of age. In most cases, the disease is caused by a growth of abnormal, non-cancerous thyroid cells which secrete thyroid hormones in excess of the normal levels. If left untreated, a hyperthyroid cat can exhibit many, if not all, of the following signs: extreme weight loss, excessive appetite in most cases, but decreased appetite in some, muscle weakness, heart disease (increase in the size of the heart, increased rate, changes in heart rhythm, cardiac arrest), intolerance to stress, and eventually death.

In most cats with hyperthyroidism, I-131 therapy will be the gold standard treatment!

How does radioactive Iodine (I-131) work to treat the hyperthyroidism?

The thyroid gland is the only tissue in the body that concentrates iodine actively. Therefore, the radioiodine is concentrated by the hyperactive, abnormal thyroid tissue. Because the normal thyroid tissue becomes atrophied (decreased in size and ability to function) in the hyperthyroid patient, the normal thyroid tissue itself should not concentrate the radioiodine.

Cats that are suitable for I-131 therapy

  • Otherwise healthy cats with Hyperthyrodism (occasionally cats with concurrent disease can be treated but suitability needs to be determined on a case by case basis after discussion with your veterinarian)
  • Cats which are comfortable being separated from their family for 1-3 weeks

Cats who may not be suitable for I-131 therapy

  • Cats who become anxious or aggressive in a vet clinic or boarding cattery situation
  • Cats who require daily medication or who have unstable medical conditions
  • Cats who have chronic gastrointestinal issues or inappropriate urinary issue

How is the radioiodine administered?

The protocol for administering radioactive iodine at the Auckland Veterinary Hospital is by the subcutaneous route (under the skin). Because of the nature of the injection, we generally will administer a small amount of sedation. This is to ensure the safety of both the patient and the staff members involved in the injection procedure as well as to ensure the entire dose is administered to the patient.

How long will my cat have to stay in the hospital after the treatment?

The hospitalization period varies depending on the dose of I-131 your cat receives. Does is determined by the level of thyroid hormone in the blood at the time of diagnosis and the severity of clinical signs. Most cats will stay 7 days in hospital, but we can organise a longer stay if required.

How will my cat be cared for during their stay?

Your cat will receive attention daily from one of our veterinary nurses. During this time your cat will receive routine care (feeding, watering, litter change, cage clean-up). To minimise exposure of our staff to radiation we do not accept cats for I-131 therapy if they need daily medication or extra care.

We encourage you to bring in something familiar from home for your cats (i.e. blanket, shirt, etc. with their scent on it), this helps the cats settle. However, this does need to be with the understanding that it cannot be returned after the stay because of radiation safety considerations.

We encourage you to bring your cats usual food from home so we can feed it for the duration of their stay. This is to avoid sudden changes in diet which can cause gastrointestinal upset. If food is not bought with your cat, we will provide a prescription diet at additional cost.

What happens once my cat is released from the hospital?

For 2 weeks after your cat returns home there will be some residual radiation present at low levels. To keep your human family members safe, you must follow these rules:

  • Your cat must be kept indoors to minimise the risk of radiation exposure to other people
  • No children under the age of 18 or pregnant people should be involved in the care or come into close contact with the treated cat
  • You must not allow your cat to sleep in the same room as a person or sit on their lap. Limited (<30mins) patting is allowed
  • Restrict access from food preparation areas
  • Hands must be carefully washed after handling the treated cat or their excrement
  • Used litter should be kept in an airtight rubbish bag away from occupied areas. This rubbish bag can be disposed of in normal waste at the end of the 2 week period
  • Treated cats must not be allowed to toilet in vegetable gardens

If you are not able to follow these restrictions for 2 weeks after the release of you cat from hospital an additional stay in our cattery can be arranged (at an additional cost)

What type of monitoring should be done post radioiodine treatment?

Thyroid levels and kidney enzymes should be measured at 1 month and 3 months after the radioiodine treatment. This helps monitor for kidney disease which can be unmasked with treatment for hyperthyroidism. Please note: radioiodine therapy itself is NOT associated with causing kidney disease in cats.

During the first 3 months following treatment, it is possible for patients to experience a period of subclinical hypothyroidism (low thyroid levels where the patient shows no signs of illness) that is almost always symptomatic and does not require therapy in the majority of patients. In most patients, the atrophied (decreased in size and functional ability) thyroid tissue becomes functional, and the patient’s thyroid level returns to a more normal level. Cats that continue to have low thyroid levels may require thyroid supplementation.

Patients that continue to have high levels of thyroid hormones by 3 months after the radioiodine therapy will likely require re-treatment. This has only been noted in approximately 5% of cases.

Please don’t hesitate to get in touch with your veterinary clinic to discuss more about I-131 treatment and your cat

Available at Auckland Clinic Only

Vets please use the form below to refer a case.
For further information or to discuss case suitability please email

Note: an actual T4 reading at the time of diagnosis is required for accurate dosing, please contact the clinic to discuss if you do not have this information

CS at time of diagnosis

Ie. Weight loss, Increased appetite, Pu/Pd, vomiting, diarrhoea

Palpation of the thyroid gland

Note: we require these tests to be repeated within the 30 days prior to admission. If a date is booked longer than this away the tests will need to be repeated while the cat is still on their thyroid medication.

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