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My cat has hyperthyroidism. Is I-131 better than methimazole?

Updated On September 23rd, 2025

My 16 year old cat has been on methimazole for hyper thyroidism. She currently takes 3.75 mg/day. She has been taking it for about 9 months and her dosage started at 2.5 mg/day. Our vet suggests continuing with this therapy indefinitely, but I see some info online that radiotherapy has better results and we could pay for that if it’s substantially better. Looking for a second informed opinion. Does methimazole have any advantages over radiotherapy? Thank you, David

3 Answers

Most Helpful Answer

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Answered By Lauren Jones VMD

Veterinarian

Published on January 23rd, 2021

I-131 (radioactive iodine) is a fantastic option if your cat's T4 (thyroid hormone) and kidney values on bloodwork have remained stable while she's been taking methimazole. In fact, it's considered the gold standard treatment modality in cats that are considered good candidates. There are some cats that have existing kidney disease "unmasked" once treatment for hyperthyroidism begins, so these cats are not considered good candidates for I-131 therapy. Hyperthyroidism is a condition that requires lifelong management, so your vet is correct in stating that treatment should continue indefinitely. However, you're correct in that you may have another good option. Methimazole is not advantageous over I-131. If anything, it has more potential side effects than I-131, but we use it since the good far outweighs the bad for the vast majority of cases. Few specialty hospitals actually administer I-131, but if you have one in your area, it is absolutely worth scheduling a consultation to help determine if your cat is a good candidate for this treatment modality. I hope that everything goes well!

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    Answered By Dr. Melanie, BVSc MS

    Veterinarian

    Published on

    Hi David, great question! Personally I think that radiotherapy is a great option, especially since you don't have give a cat a pill twice a day! According to the Cornell Veterinary School, "The advantages of radioactive iodine therapy are that the procedure most often cures hyperthyroidism, has no serious side effects, and does not require anesthesia. It does, however, involve the handling and injection of a radioactive substance that is only permitted at facilities specially licensed to use radioisotopes. The radioactivity carries no significant risk for the cat, but precautionary protective measures are required for people who come into close contact with the cat. A treated cat has to remain hospitalized until its radiation level has fallen to within acceptable limits. Usually this means that the cat will need to be hospitalized for between three and five days after treatment. Because of strict treatment guidelines, most facilities will not allow visitors during this quarantine period." If you can afford it, I'd discuss with your vet about possibly getting radioactive therapy done. I hope this helps.

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    Answered By Linda G, MS, DVM

    Veterinarian

    Published on April 13th, 2018

    Hello. If you can do the radiation therapy, this is considered the gold standard for the treatment of hyperthyroidism in cats. With this treatment, there is no more pill & no need to feed a special diet. Any particular concerns should be directed to your veterinarian since she has all the particulars on your cat, but this is often the very best way to go. Thanks for using Petco Pet Education Center, formerly Petcoach.

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