Radioactive Iodine for Feline Hyperthyroidism

Updated November 28, 2020

If you have a cat diagnosed with hyperthyroidism you’re not alone. Studies indicate that around 2% of cats are affected, and up to 10% of geriatric cats.

There are many treatment options available for thyroid problems in cats. However, the one that generates the greatest discussion is radioactive iodine. We don’t offer it, but we can refer you to an appropriate centre.

First, here’s what you need to know.

What Is Radioactive Iodine Treatment?

Iodine-131 or 131I is a radioactive isotope of iodine with a half life of 8 days. Like stable iodine, it is naturally concentrated in the thyroid gland. Therefore, it has two claims to fame:

  1. A serious health hazard after nuclear accidents such as Chernobyl or Fukushima
  2. An effective treatment for thyroid diseases

131I works by killing thyroid tissue and reducing the amount of thyroid hormone being produced. The effect is usually permanent.

How Safe Is Radioactive Iodine?

While low doses are known to cause thyroid cancer, therapeutic doses, especially in cats, are extremely safe. The main safety considerations concern over-treatment and kidney disease.

Over-treatment occurs when the 131I kills too much thyroid tissue. This puts cats into a state of insufficient thyroid hormone, or hypothyroidism. A recent study found that 40% of treated cats were visibly hypothyroid, and another 13% hypothyroid when tested. This contrasts with a 1995 study that found only 11% of cats became hypothyroid.

Hypothyroidism matters because it’s unpleasant, as anyone who’s been affected will know. However, it also matters because these cats are more likely to develop kidney problems and have shorter lifespans.

Kidney disease occurs due to a reduction in blood flow to the kidneys when thyroid hormone levels are lowered. Most of the time, these older cats already have renal insufficiency that’s being masked by the artificially high thyroid levels.

Therefore, kidney problems can occur even when the post-treatment thyroid levels don’t drop lower than normal levels. However, the lower they go, the greater the risk.

Radioactive Iodine Treatment Advice

Based on this, here are my personal opinions for safer and effective use of 131I.

Do A Pretreatment Trial

Even if you don’t want to use carbimazole or methimazole tablets, it’s a good idea to trial these for a month before giving 131I treatment. Antithyroid medications are reversible, so if you detect the development of kidney disease, you can choose to stop or reduce the dose. Once the 131I is given, there’s no going back.

After 2 to 4 weeks on antithyroid medication, your cat will need a second blood test, including urea, creatinine and SDMA levels.

Ask About Lower Radioactive Iodine Doses

A recent study showed that reducing the dose to 74 MBq [2.0 mCi] resulted in no extra treatment failures but significantly fewer hypothyroid cats. This is roughly half of many current doses. In the study, this was done only for cats with pre-treatment thyroxine levels under 167 mol/l or 13 µg/dL.

Talk this over with your vet. They may have very good reasons why this study won’t apply to your cat.

Treatment failure is documented to occur in around 5% of cases. These are cats whose thyroid hormone does not normalise. You then have the choice to try again, or pursue another option.

Follow Aftercare Advice

Studies show that the number of cats with hypothyroidism increases over the 6-12 months after treatment. Therefore, your vet will recommend follow up blood testing, and you should definitely get this done. Testing for both T4 and TSH is more expensive than T4 alone but considered to be more reliable at detecting hypothyroid cats.

If your cat develops overt or subclinical hypothyroidism, he or she should be treated with thyroid hormone supplements. It’s certainly disappointing that you will still need to give a medicine, but talk over the options with your vet. The survival advantages are clear.

Radioactive Iodine Treatment Locations

Most veterinary specialist centres and even some private vets offer 131I treatment. Wherever you live, there will probably be a vet nearby able to offer this service. In Adelaide, these are:

You will find that your cat is expected to stay in the hospital for a week while they excrete radioactive iodine. After this, follow your vet’s advice closely.

How Much Does Radioactive Iodine Cost?

The fee for treatment including the hospital stay is around $2000. After this, you should be able to get follow up testing done at your local vet.

Have something to add? Comments (if open) will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. Meet his team here. The information provided here is not intended to be used as a substitute for going to the vet. If your pet is unwell, please seek veterinary attention.

References & Further Reading

Fernandez, Y., Puig, J., Powell, R., & Seth, M. (2019). Prevalence of iatrogenic hypothyroidism in hyperthyroid cats treated with radioiodine using an individualised scoring system. Journal of Feline Medicine and Surgery, 1098612X18822396.

Lucy, J. M., Peterson, M. E., Randolph, J. F., Scrivani, P. V., Rishniw, M., Davignon, D. L., … & Scarlett, J. M. (2017). Efficacy of low‐dose (2 millicurie) versus standard‐dose (4 millicurie) radioiodine treatment for cats with mild‐to‐moderate hyperthyroidism. Journal of Veterinary Internal Medicine, 31(2), 326-334.

Peterson, M. E., & Becker, D. V. (1995). Radioiodine treatment of 524 cats with hyperthyroidism. Journal of the American Veterinary Medical Association, 207, 1422-1428

Peterson, M. E., Nichols, R., & Rishniw, M. (2017). Serum thyroxine and thyroid‐stimulating hormone concentration in hyperthyroid cats that develop azotaemia after radioiodine therapy. Journal of Small Animal Practice, 58(9), 519-530.

Peterson, M. E., Varela, F. V., Rishniw, M., & Polzin, D. J. (2018). Evaluation of serum symmetric dimethylarginine concentration as a marker for masked chronic kidney disease in cats with hyperthyroidism. Journal of Veterinary Internal Medicine, 32(1), 295-304.