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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

Lauren Jones VMD

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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Related Answers from Veterinarians

  • Answered By Dr. Melanie, BVSc MS

    Veterinarian

    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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    1Pet Parents found this answer helpful

Smart Picks for Smart Questions

Health & Wellness
My cat's hyperthyroidism: Methimazole side effects. What to do?
Luv Bug is so very fortunate to have you as an owner. I am sure that you are one of the main reasons that she is going strong at 17 years of age. Kudos to you for your consideration and compassion for your pet. In terms of hyperthyroidism in cats, there are only three real treatments on the market today. These are methimazole treatment, thyroidectomy surgery, and radioactive iodine treatment. It is believed in the veterinary community that radioactive iodine is the gold standard treatment today. With the information you have provided, I can offer you a few possibilities for future treatment: 1. Perhaps you can get the math him is all in a tablet form to crush it into a fine powder and then mix it with any palatable substance that she likes and then squirt it in her mouth with a syringe. This, however, may cause some behavior problems because she may not like this either. 2. If you are willing to accept the costs and all of the different parts that go with the radioactive iodine treatment, I say go for it because the success rate for that specific treatment is excellent. Thank you very much for your question and I know you will make the right decision on her behalf. Have a great day.
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My cat has hyperthyroidism. Is A131 safe? How long is quarantine?
The radiation is good treatment option, however it's important start with the oral pill (methimazole) first to make sure his body tolerates having lower thyroid levels first. When we get the thyroid disease under control sometimes we uncover kidney disease that was always there. The other down sides of A131 are cost, availability of the treatment (you have to go to a facility, this isn't something your vet can do), and the process (needing to stay at the facility, I believe it's 3 days but you'd have to check on the protocol at your local facility).
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Any long-term side effects from radioiodine for cat hyperthyroidism?
Usually cats do well with the radio active iodine therapy. There is a chance for them to become hypo thyroid meeting they would need thyroid supplementation after the treatment. But generally speaking most cats do very well and no further treatment is needed. However your veterinarian will need to make sure there's no other underlying issues such as early kidney disease prior to doing the radio active iodine therapy. If they do find other issues then your cat may not be a good candidate for that treatment but other treatments such as daily medication or even surgery to remove part of her thyroid gland might be options.
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What are the best medication options for my cat's hyperthyroidism?
I assume that Chetoh was diagnosed with hyperthyroidism, as cats are rarely diagnosed with hypothyroidism. Typically, I treat with methimazole pills twice daily as a starting point. I find that the T4 hormone is better controlled with oral medications than the transdermal medications on the ear because absorption is variable with this method. Once hyperthyroidism is well controlled and it is apparent that the kidneys are stable, it can be worth considering I-131 (radioactive iodine) treatment, as it stops the need for medication at home in most cases. All of that said, there are still plenty of cases that I do prescribe the transdermal ear version of methimazole because some cats are extremely difficult to medicate at home. Just make sure that Chetoh follows his vet's recommendation for follow up lab testing so that his medication dosage stays appropriate.
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Should my hyperthyroid cat get surgery or stay on medication?
Surgery will be a definitive option and treatment won't be needed anymore in the long term. Unfortunately the down side of it is the cost and invasiveness of procedure. Medical treatment on other hand requires constant monitoring and continue treatment but it is a good option as well. The decision is very subjective, but usually where cat and client are right candidates, I tend to suggest surgery. Please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.
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My cat won't take thyroid meds. Are there easier ways or radiation?
You can use a transdermal gel to treat Max's thyroid condition. This gel is applied to the inner ear and is absorbed through the skin. Discuss this option as well as radioactive iodine Therapy with your veterinarian. There is a company in Stamford, Connecticut that performs the treatment. Follow the link for more information. http://m.thyrocat.com/index.php
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My cat has hyperthyroidism. Is 4.8 early? Can I use natural remedies?
In the majority of cases hyperthyroidism is due to a benign tumour (adenoma )within the thyroid gland. As time goes by, the lump continues to grow with worsening of the symptoms that will require more treatment. It is difficult to be stable on the medications, they wil need adjusting and repeat blood test. There are other more permanent treatment options such as radioactive treatment. I am quite positive about integrating conventional therapies with holistic ones i general, in this case though I don’t see how it would help .
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