The only "cure" for hyperT is radioactive iodine (I-131), which cures the disease in 98-99% of cats. It is definitely the hyperT treatment of choice for cats with healthy kidneys and would certainly be the best option for any cat who can't tolerate thyroid medication. You can read about 1-131 at the link below.

Surgery is the most risky and perhaps least effective treatment for hyperT, since it may not remove all of the diseased tissue, causing the disease to remain or return after surgery.

As far as methimazole is concerned, exactly what "very low dose" caused your cat to have the bad reaction? It's possible that you may be able to cut the dose back even more and see how that works for your cat. Other options are to try the transdermal form of the drug (a gel that gets rubbed into the inner ear flap) or to try carbimazole (which converts into methimazole only AFTER it leaves the stomach). I don't know if either of those options would reduce or eliminate your cat's negative reaction to methimazole, but they might be worth trying. If these other med options don't work for her, then I-131 is definitely something to look into.

I doubt if you'll be able to put any weight on your girl until you successfully manage or cure her hyperT.

Laurie

Here is a link that might be useful: radioactive iodine treatment for hyperT in cats

I opted for the radioactive iodine treatment for my cat a few years ago.

Emotionally, it's horrible because your cat is isolated from you and everyone else for a couple of weeks, but, in the long run, I think it's the best option.

When your cat gets home from the treatment, she'll be back to her old self in no time.

I'd look into it. I chose it because of all the side effects I'd read about with the medications.

Feel free to email me if you want to know about my (my cat's) experience with the treatment.

Time of isolation varies depending on the state regulations. Here in NC, you can get your cat back in 3 days, with some precautions to take regarding handling of slightly radioactive urine and stool. The I-131 treatment is just a single subcutaneous injection, not painful. Definitely look into it.

thanks for all of your replies. I believe my vet did mention the long term isolation time with the rad. iodine treatment, so it must be a few weeks in my state as well.

The dose for the methimazole started out at 2.5 mg, to be increased to 5 mg eventually, but I never got past the 2.5 dose the first time, after a few weeks is when the reactions started. We stopped the med, gave her system some time to clear out, then I started her again at a little less than a quarter tab ONCE a day...which would be I guess around 1 mg, give or take. The idea was to start her out at much lower dose and very very slowly increase her...my vet said she had another patient who slowly acclimated her cat to the med this way and that it worked out fine, her cat's body adjusted well to the slow and gradual increases and she was able to eventually give her a full dose on a regular basis. I was hoping this would work with Liza, but after about 10 days, she began having the same skin reactions, though not as severe, on the roughly 1 mg dose. I suppose I could try again with half a mg.

I really do hate to have to be separated from her for so long with the rad. iodine treatment, but if she can't take the meds in any form (going to try the gel and the carbimazole if possible) it may be the only option, or her health will just deteriorate. Though she has high blood pressure, the vet said that all her systems seem to be pretty good for her age, so maybe we can do that. I have some thinking to do, and also some new ideas to discuss with the vet.

Thanks Laurie for the link, I am going to read that now. And thanks to everyone else as well, I appreciate your time to respond!

Jen,

Which state are you in? I'll be happy to do a Google search and see what I can find out about isolation times for I-131 treatment of cats in your state. As difficult as it would be to be separated from your girl for I-131 treatment, the long term benefits for her and for you would be well worth it.

Methimazole and carbimazole are very serious drugs with very serious potential side effects both over the short and long terms. Even if your girl can acclimate to the drug over the next few months, these drugs have the potential to cause permanent liver and kidney damage over the long term. You need to keep that in mind when weighing your treatment options.

Just so you understand, there is no "full dose" of methimazole per se. The proper dose for your cat is whatever dose maintains her T4 hormone at the proper level. And since hyperT is a progressive disease, it's common for the dosage to need to be increased over time as the disease worsens. The safest strategy for determining the proper med dose is to start very low, maintain the low dose for 3 weeks, then retest and see what effect that dose has had on the T4 level. If the T4 level is not yet optimal, the dose is adjusted up or down slightly, as necessary, with another retest in 3 more weeks. That schedule of med adjustment followed by a retest in 3 weeks is maintained until the cat achieves euthyroid (normal T4 level), after which the cat is maintained on whatever dose achieved euthyroid. Retests are then performed every 3-6 mos to make sure the dose is still maintaining euthyroid. Many hyperT cats maintain euthyroid on very low doses, some as low as .625 mg daily. The goal is not to work your girl up to 5 mg a day. The goal is to achieve euthyroid at as low a dose as can do so.

Laurie

Laurie, I am in New Jersey, southern half of the state. Burlington or Camden or Ocean County are convenient. Philadelphia is the closest large major city.

Thanks for taking the time to share this information with me. Hopefully it will also help others in the future if they have the same issue. My vet, I suppose because of the cat's weight, started me at 2.5 mg, but the bad reactions occurred before I got her back in to test her levels (to see if the dose was therapeutic). I was thinking of going down to one quarter of that dose-which would be .625 as you mentioned above- as a last resort with this med. However, what you say about the long term effects of this medication leaves room for more thinking about it. She is an older cat, and I don't want to do any damage to her organs.

So yes, if you could find out what the quarantine time is in NJ, I would appreciate that, or if you could tell me where to search for such info, that would be fine too.

You have been very helpful, and I am grateful. Do you work in the animal field?

I ran a Google search for I-131 treatment clinics in NJ, and it appears that you have quite a few of them. Most of them note a post-treatment hospitalization quarantine of only 4 days. One of them notes 4-7 days. So you've got a nice short quarantine period there in NJ.

The only "animal field" work I've done is as a riding instructor at my own small stable for a number of years. I also majored in Equestrian Studies in college several decades ago. Other than that, my animal education and experience has been provided by my rather extensive 4-legged family over the last 50+ yrs. There's not much they haven't taught me about.

Laurie

Jen-
I am going through the same thing right now with my 12 year old cat. She currently has a cone on her neck to prevent her from itching her eyes. They think it's from the Methimazole, but I stopped giving it to her a week ago. Her eyes are puffy and itchy and don't seem to be getting much better. Could you tell me how long it took for your cat to stop these symptoms after you discontinued the Methimazole? She is supposed to have the radioactive iodine treatment in less than two weeks, but if she doesn't get better soon she won't be able to have it yet. By the way, after doing so much research, I definitely recommend that treatment. It looks like the life expectancy of the cat is quite longer with the treatment than it is with the medication. I am so worried about her being isolated during treatment, but I think it will be best in the long run.
Will you please email me as well as post because I am not going to be checking this site as it was just an internet search. Thanks! ashleyyyy@hotmail.com

Laurie, that is GREAT!! If it is only a few days, or even a week, then I think that both I, and the kitty, can deal. I have her on .625 mg now, but I have alread noticed some fresh scratches on her ears...

Ashley, I feel for you!! So hard to see this. My cat on a higher dose had red puffy eyes, a scratch from the corner of her eye down to the tip of her nose, scratches all over her neck and ears, plus she is pure white so all of the usually pale pink skin you can see on white cats, such as their ears, would turn beet red. After her worst reaction, the vet gave her a shot of hydrocortisone and meds for me to give her at home and told me to keep her off all other meds, such as even low doses of the methimazole for about two weeks, so I believe it can take some time for the allergic reaction to subside and for them to get back to normal.

Aren't we the lucky ones to have one of those cats that fall into the 1% that have a bad reaction to this med!

Your cat should be ok soon, did you vet give her anything to ease the reaction? If not and she still seems to be suffering, I would ask. Please let me know how it goes with the iodine treatment.

And Laurie, thanks again. You have been a great help.

Jen & Ashley
I know this is somewhat an older post, but I have a few questions for you.
Peachy, my 14 year-old cat has been on Methimazole (1.5 mg a day-liquid) for 6 weeks. She will have her first post-med blood work lab on Monday. Remembering your experience with this drug, I was keeping a close eye on her. Last week, there was a scratch over the eye that healed. Today, I noticed a small bald area just at the lower corner of her eye. Do you remember if that's how it started for your cats? Not bad at this point, but I don't want it to get any worse.
Hope you both found an acceptable solution for your cats (besides the e-collar). Iodine treatment?

Hope you are doing well,
Anne-Marie

Hello, my cat Gabby was recently put on Methimazole for her Thyroid. All seemed to be going well but within the last few days I have noticed scratches above her eyes and in her ears. She also seems to be biting at herself like she itches all over. The vet did have her on 1 tablet per day but when she was not gaining enough weight upped it to 1 1/2 pills per day. When I noticed her puffy eyes and lackluster attitude I backed it down to 1 pill per day on my own. This morning I found her just sitting in the corner by the water bowl. She didn't sleep with us all night. I do have a call into my vet. In Wisconsin how long is the period of time she needs to be quarantined if she gets the Iodine injection. Thanks to anyone who can help. Ifeel so bad for Gabby that it makes me want to cry. PLEASE HELP!!

Maureen,

Scratching at the face is a known side effect of methimazole in some cats. If one of my hyperT cats had a similar reaction (which, thankfully, none of them have), I would stop methimazole immediately and discuss alternate treatments with my vet. Radioactive iodine is the only cure for hyperT, so it is the treatment of choice. Below is a link to the I-131 informational brochure of the UWM vet clinic.

Laurie

Here is a link that might be useful: UW Madison I-131 brochure

My little cat was on Methimazole (1/2 of 5 mg 2x/day), but she was always throwing up, so I stopped giving it to her. She seems to be fine now, except that she is so thin. She'll go outside in the morning sometimes, eat grass and throw up then, but seems to be keeping what food she does eat down. Unfortunately, I had the iodine treatment scheduled, but had to cancel when I lost my job. Likewise, I can't afford to take her back to the vet. Should I reduce the Methimazole to 1/2 pill once a day - maybe 1/4 pill 2x/day?

@laurief_gw I would like to talk to you about information on carbimazole. my cat is on methimazole and i am interested in carbimazole producing less side effects because of the way it is processed that you note above, I am having a difficult time finding sources to cite that discuss that.

joannamarie. All i can tell you is be very careful. My vet used methimazole. I followed her directions to a T. I really believe if i gave the cat the meds less.....The results would have been much better.

I had to sign in to comment on this because it's the stupidest thing I've heard that they quarantine your cat after treatment of radioactive Iodine. I personally had Hyperthyroid and was treated with radioactive iodine and they didn't quarantine me. They just said to stay away from young children. Why would a cat on a much smaller dose be totally quarantined??

There are extremely strict laws and paranoia concerning radiation, so this is just the simplest way to appease the public and CYA.

CYA? lol

I was a Radiation Worker for over twenty years. There are simple common sense requirements for dealing with the cat after receiving the I 131 treatment.

By no means are these steps "CYA," just ways to minimize any exposure and avoid risks. In my area, trash and waste is subject to radiation exposure. I was required to switch cat litters to a (toilet) flushable brand as had I double bagged it and sent it to the dump it may have triggered serious security concerns - or do some think that detecting radioactive material in trash is simply more "CYA?"

While some amount of common sense is always appropriate, to dismiss minimizing unnecessary radiation exposure is somehow simply "CYA" shows a total lack of understanding of the issues.

Following the post-treatment protocol for treating a cat with radioactive Iodine should be a safe and effective process with minimal risk of health damage to the owners and veterinary staff.

Do you know what the maximum (that is if you ignore your veterinarian's warnings and expose yourself to your pet's urine and saliva) risk for radiation exposure to your cat is following a standard I 131 treatment? It is less than 1 mSv (aka "millisievert", a standard medical dosage measurement of radiation). In perspective, the average US citizen is exposed to over 3 mSv yearly IF they have no x-rays, transcontinental flights, do not live at high altitude or have any CT scans (all which can expose a person to an additional 2.5-3 mSv per year). In reality, the average American citizen is exposed to about 6 mSv a year. mSv exposure is a concern as radiation exposure has been linked to the development of cancers (as well as other forms of tissue damage). But the increased risk of developing cancer from exposing oneself to radioactive cat urine following a typical I 131 treatment would be less than 0.01% over the remainder of your life compared to just living a normal life without this situation. Though no one knows for sure how much just living in the US and being exposed to such low level radiation, as we all are, causes people to develop cancer, but it is thought to be very little.

Radiation exposure is far more dangerous to children and unborn embryos and fetuses which is why your doctor will tell you to stay clear of them until you are less of a threat to them, should you have gone such a treatment yourself. But just telling a cat owner to avoid exposing their children or unborn children to radioactive cat urine is not going to hold up in court in the very unlikely event something happens to those individuals and your state has mandated these cats should be kept in quarantine for 3-4 days to avoid such potential problems. Some states seem less concerned and allow cats to go home right after treatment, but most do not. The differences are more liability related rather than anything else.

So if your cat were to be treated with I 131 and this potential minuscule increase in radiation exposure were to be ignored by your vet, AND you or another family member were to develop cancer later on in life, potentially your veterinarian could be held responsible. Hence the CYA protocols to prevent such exposure to radiation AND liability (the latter being by far the more serious threat).

I am not saying that these measures taken to keep one from being unnecessarily exposed to radiation from such a procedure are worthless, but they are MOSTLY to avoid the potential liability often associated with such scary things like radiation and cancers, as the true danger of radioactive cat urine is nearly negligible. You are far more likely to develop a life-threatening infection from being inadvertently scratched or bitten by your cat than you are to develop cancer later on in life from being exposed to radioactive cat urine. But few think to sue their vet for missing a nail during a nail trim or failing to warn an owner about the dangers of being bitten by a cat with poor dental health.

Radiation safety IS important. And avoiding exposure to excessive amounts of radiation is strongly recommended by all for obvious reasons. But it is the massive amounts of red tape, government regulations (good or bad), huge costs of following such regulations, and potential massive liability that keeps most veterinary facilities from being interested in getting involved in such matters, NOT the actual dangers of radiation exposure to clients. Potential exposure to employees is an entirely different situation, however... and another reason to think long and hard before getting involved with radioactive isotopes.

What a scary and uninformed post based on Google, presumably.

What you originally said was, "There are extremely strict laws and paranoia concerning radiation, so
this is just the simplest way to appease the public and CYA"

1mSv is the annual dose limit for the general population for one entire year.

They make you leave the cat for 4 days so they can CHARGE YOU MORE MONEY. Its just that simple.

It is very bad for you to say methimazole killed your cat. Hyperthyroid killed your cat. I have lost two cats with hyperthyroid and now dealing with a third. Yes, the drug can be awful for them. I lowered the drug on my last one to come come and find her totally blind, yowling insanely, while I could do nothing to comfort her. She had most likely had a stroke and I had to put her down. She was 17 years old and this broke my heart. The only alternative to this drug is iodine radiation treatment or surgery. I did the radiation on one cat. It helped a lot. They wanted to do a second dose. They gave her too much the second time. It burned her throat because the goiter had spread into her stomach, onto her lunges, everywhere it was a record breaking size on this little cat. She was unable to take the drug because she vomited blood when taking it. The radiation scarring made her unable to swallow food and I had to put her down in the end.

That is a sad tale junecareyart. I'm so sorry for you and your beloved cat. I am on here because of a desperate search as to why my hyperthyroid cat has suddenly stopped eating and drinking. I'm suspecting the methimazole may be the culprit. I had no idea. And now I have no more words, just frustration and worry.

Keely did your cat just now start on meth - if so you might want to try the transdermal which is applied via a gel to the inside ear and absorbs thru skin less impact to the gastro intesitinal tract and much easier than pills. Also look at dose - some vets start with too high a dose. Better to start low and work up.

If shes been on a while and doing well - h as thyroid level been checked recently? High thyroid levels can in fact cause nausea - not eating. Or it could be somehting else entirely. So very sorry - my old boy is struggling with the same situation.

Our cat was recently diagnosed with hyperthyroidism and Thyronorm prescribed. She took the whole dose of 0.5 twice a day for about a week with no side effects then she stopped eating, became very lethargic and couldn't even hold water down. Back to vet for more bloods and when I suggested an allergy I was told this was very unlikely!! Well within a further 2 days her face was swollen and she was clearly unhappy, not eating, hiding etc. We are due back at the vet next week but have stopped the medication in the meantime. She seems more like herself and is eating again which is good in itself I guess. Think the iodine treatment may be the next step. Does anyone know how long the facial swelling will take to go down?

Victoria, I am not sure what that .5 is - if this is methimazole are you sure it wasnt 5 mg? If so too many vets start at 5 which is WAAAYYYY too high and would explain the side effects. Starting dose needs to be low exactly so that side effects are minimized and thyroid levels dont go too far the other direction - 1.25 mg is the recommendation and then based on the testing work up to the correct dose. If it really was only .5 mg that is a very low dose fwiw.

Yeah the I-131 is the way to go - when you add up the cost of bloodwork and drugs over the years, and the hassle of giving the meds - the couple thousand for what is essentially a CURE is quite a bargain I think!

Hello everyone and sorry for what you're all going through I am also starting this horror now no one has mentioned what the T4 level was of their cats mine cat was only 2.1 over the limit I also saw that they have a prescription felimazole for cats this methimazole it's for humans I saw it on chewy I am thinking of using that instead of starting him on the other also I believe 5 mg is 2 times a day is way too much and I will have a dead cat also he has allergies to begin with please let me know your levels thanks

We put our cat on methimazole transdermally for 2 weeks. He has completely stopped eating, vomits all the time and is clearly depressed and in pain. I am taking him off it and will try the radioactive iodine but I want him to bounce back a little before we send him off. Any idea how long it will take for these side effects to wear off? he was a happy guy before we started this medicine.

Yes the I-131 is the way to go! Not many medical procedures have those fantastic odds of having a successful outcome, and when you consider costs over a period of 5-10 yrs it really is cost effective. FYI vets often prescribe way too high starting dose and the med establishment in general does tend to minimize side effects and the very real possibility of too high a dose creating serious problems. Per more enlightened vets (incl Dr Mark Petersen DVM, the authority on feline hyperthyroidism) start low at 1.25-2.5 mg per dose, 2x per day and then work upwards based on testing ev 4 wks until correct dose is found. Sometimes the low starting dose IS the correct dose. Felimazole is methimazole - just marketed and dosed for cats. Transdermal makes it much easier to adjust or lower dose. Don't need vet approval, just do it but then let them know when your cat is tested what dosage youve been using, so they know. There is a pretty good group on groups.io called Feline Hyperthyroid Support.

Hello to everyone here. I am not sure if any of you will see this, but my cat is experiencing something similar to Victoria...facial swelling... after stopping Felimazole. Was there an answer given to how long it took fir the facial swelling to reduce?

Yes I don't understand about the dosage shouldn't it go according to the T4 level if you start off high what do you go by I've been giving my cat a quarter 2 time day of the methimazole tried the felimazole didn't work out it seems okay for now but I am seriously considering injection it's too much stress now I'm back and forth to the vet check the level still working on the dose

Our guy has been on the trans dermal methimazole going 3 years now. Expensive. Just increased his dosage to 5mg 2x a day, he's sickly from this. Going to try to scale back to find a happy level. However vet says tsh was 15 extremely high. Hate having him go through this. His other blood levels all in normal range. Vet seems inexperienced with this condition. Feel horrible for my cat. You can tell he's ill.

Sorry to hear about your guy after putting my cat through all different test for the i-131 treatment I was informed that there may be something underlying and then my vet included he doesn't have the symptoms for hypothyroidism only the blood work I am back to square one I am taking him to an allergist dermatologist and then find a different vet to me 5 mg two times a day sounds like a dose for a dog I have taken him off the medication until I find out what the heck is really going on a second opinion sometimes could help good luck and let me know

Do you mean hyPER thyroidism? Cats very rarely if ever are hypo, and their hyperthyroid symptoms arent necessarily the classic eating/drinking voraciously and losing weight. In fact for some its the opposite. My cat was just picking at his food and very gradually losing weight over a period of time, By the time he started acting really hyper and distressed his T4 was through the roof (and finally diagnosed then). You dont want it to get to that. Its a horrible disease, very distressing to the animal.

Re dosage, a lot of these horrific symptoms could be minimized or eliminated by starting at a LOW dosage and then raised only based on bloodwork. Guessing is not a good way to do it. My other cat was recently diagnosed, the vet said give him a low dose of 2.5 mg twice a day. Well I gave him HALF that - and amazingly, his T4 came down to a perfect range and he is now being maintained on this tiny dose of 1.25 mg twice a day. Start low, go slow.... Yeah meth is a harsh drug, but like any drug - you have to get the right dose.

i was giving him the low dose 1/4 2x's a day of 5 mg-he just wasnt himself so i decided for I -131 and i was informed as i mentioned they say something underlying ??? did your cat scratch himself/her and seem lethargic depressed. once the skin issue is diagnosed i will take it from there. just hate to give that med as i have noticed a reaction. i will have no choice since the I-131 is not an option. you didnt mention your cat's T4 levels ???

Just to be sure Im understanding, you were giving 1/4 of 5 mg., or 1.25 mg twice per day? How high was his T4 - up to about 4.9 is normal. High normal maybe, but still normal. T3 is part of diagnosis too, but Idont really understand what all goes into it. https://groups.io/g/feline-hyperthyroid/ has some very knowledgeable people and could help decipher the lab work and where to go from here.

hi, i assure you i am taking the best possible steps. there is more what is going on with him and too much to get in to . thank you

Has anyone been attacked by their cat when on methimazole or if the cat has anxiety

My cat passed away after her first 2 doses. She died in 1 second out of nowhere. I don’t know what happened or why but that medication terminated her.

Debbie downer—.5 was for my cat.

how much did she have? i was thinking of increasing like vet wants-maybe he isn't getting enough or maybe his head hurts from this drug!!!

my beautiful cat is horribly sick on methamazole anemic, fever, liver injury, lethargic, dehydrated all side effects of this poison. The vet is trying to help with fluids, antibiotic,

bone marrow stimulent. steroid. and B vitamin injection. I give her appetite stimulant to nourish her and try to fix some of the damage. Not sure if she is going to make it. i will take my chances with the thyroid rather than give her this poison.

i have a funny feeling that stuff is harming my cat and not really sure if he has hyper or something besides (as they say) kidneys going on. this on and off with this drug is not good either-but after 3 weeks on he won't eat. i am in the process of having an appointment with an endocrinologist to find out some answers. so sorry, for your kitty to be suffering so. and you are too. let me know how it is going. i hope you both are well!

My vet told me that hyperthyroidism can mask kidney disease. The increased blood flow when thyroid levels are high helps the kidneys. When levels come down with medication the kidney disease shows more.

Loss of appetite is also a side effect of methimazole. My 21-year-old has days where he won't eat anything but Fancy Feast.

In addition to the methimazole, I'm giving him Thyroid Support Gold and Kidney Support Gold from Pet Wellbeing. Both with vet approval and he is doing pretty well given his age.

It took a few month to get things on even keel.

My 14 yo cat was on methimazole (2.5 mg 2x day). it caused breathing problems/allergy attacks. His breathing continually got worse and worse thru months on meth. He was always at the vet for the breathing issues, they said he developed bad allergies, started him on prednisone. Finally he couldnt eat anymore due to his breathing issues. I stopped the meth and in 1 week his breathing was fine! I absolutely blame the merhimazole for almost killing him. He’s much better now- not taking methimazole ever again!

That stuff is poison!!! It was killing my cat l would give it on and off thank God he wouldn't eat his breathing and he was just miserable also the scratching forget the dose they say to give you'll have a dead cat in no time believe me as scary as it all sounds the best way to go is the I-131 I did and we were privileged to have it done by the most wonderful doctor himself doctor mark Peterson in Manhattan the doctor that created this treatment good luck do research

I lowered my cats dose of the methimethimazol, so far seems to be working for her. She went from curled up in a corner not eating or drinking for days to somewhat back to herself but not completely. She has experienced some drastic hair loss on both her front legs and paws though. The vet said she had not heard that much hair loss is a side affect from the drug. Anyone had this happen to their cat on this drug?