My cat has a bunch of preexisting medical problems including chronic sinusitis and new superficial masses on her belly, and when I took her to the vet a few months ago, he ran a T4 which was elevated to 7.8 (previously at 4.6). He started my cat on Methimazole liquid formulary, and she had been on it for 6 weeks, because he says we can't do imaging and go under anesthesia until her thyroid hormone gets under control. . Since starting this medication, my cat has gotten markedly worse, not eating, hiding, lethargic, vomiting, unable to jump or climb up steps. I asked the vet to switch her to the transdermal form thinking this would make her stomach less upset by the medication, but now she is even worse, she is walking sideways, can't walk more than a few steps without stopping, is incredibly weak, can't climb stairs, is vomiting and isn't eating AT ALL. She does not want to be touched or pet at all, and is hiding now more than ever. The vet never warned me about the side effects or allergy profile of this medication. Thank God I found this website because all of the descriptions of side effects are exactly what she is experiencing. I'm discontinuing the medication immediately. I've been distraught for the past week, because essentially I have never seen like this and feel that she is declining rapidly. I was going to discontinue the medication but was reticent to because if her repeat T4 is too high they won't let her go under anesthesia for the imaging (trying to rule out possible mammary gland tumor and look for mets). I would have never started her on this if I had known this would be the reaction. I am going to talk to the vet about switching to the food, but I just hope these side effects are reversible and I haven't permanently hurt my cat by putting her on this medication.

My cat Rascal had been on Methimazole twice daily 5mg. He stopped eating and is cold to the touch. He stares as if he doesn't know where he is and has difficulty using his hind legs. I took him to the vet and she said she thought it was his kidneys because the test came back with low creatinine. This perplexed her because she said normally kidney problems present with high levels. He is drinking a lot of water now. I would not recommend the use of this drug. L

I forgot to add that Gary had a rapid heart rate and a murmur, accompanied by weight loss, increased thirst and urination, strange behaviours. He would go into the closet and scratch at the wall as well as trying to burrow under me when I was sitting on the couch. Sometimes he would binge on water and then vomit it up. I think he was ravenous for more food as he was begging constantly. This behaviour has decreased since he was started on the methimazole.

My 12 year old cat, Gary was diagnosed with hyperthyroid and mild renal impairment about two months ago. He started on methimazole 2.5mg twice daily via transdermal gel and then I decided that since his appetite was still good I would switch to the chews. He has been eating well and gaining weight but recently developed some itching in his neck and scratching until he has sores. He also seems to feel cold, with cold ears and feet-I'm snuggling him right now as I type this. He has vomited a little but not enough to worry about, maybe once a week. I am considering the radioiodine treatment as his kidneys are impaired and the surgery seems like the least favourable option. Has anyone else done the radioiodine treatment who would share their story with me? Good luck to all with treating their feline friends for this awful condition!

My 5 year old cat Nala was recently diagnosed with hyperthyroidism. She was prescribed 5mg of methimazole 2x a day. After about a week, she began vomiting up everything that went in her body and refusing to eat or drink. She also seemed weak and lethargic. Ended up having to take her to urgent care and after doing several tests to rule out other possibilities, the on-call vet said that she most likely was having a bad reaction to the medicine. I quit giving it to her and she's been getting back to her usual self. After reading everyone else's experiences, I never want to give her methimazole again. Hoping that the radio treatment or surgery will be an option. I'm upset that I wasn't told about these side effects from the get-go and that my kitty had to suffer because of them.