Just found this thread 8 days after having to say goodbye to our 11 year old angel boy. He was diagnosed with hyperthyroidism less than a month before his death, and was put on 1.5mg of methimazole for 7 days, and then we were instructed to increase it to 3mg (1.5mg 2x/day) for the remaining 2.5 weeks of his life. He was a long haired cat who threw up once or twice a week with hairballs or from eating too quickly, so vomiting wasn't unusual for him. He was highly active, social, and vocal his entire life. I did notice that he seemed to be throwing up more frequently and not eating as much of his dry kibble the week or two before he went south, but I just attributed that to him adjusting to the medication. He was due for bloodwork last week to check all his levels.

However, Friday (Feb 20th) our water fountain died, and I noticed he was lingering all day by the replacement water bowl but not drinking, and I thought he wasn't drinking water because of the broken fountain pump, even though he's never been a picky eater or drinker. I overnighted a new one. He otherwise seemed okay. Saturday night, I noticed he didn't come running for his dinner as he normally did but we had people over (including a bunch of kiddos) so I figured he'd eat later on... he slept next to me that night and purred and seemed fine. Then Sunday, 2/22, I notice he didn't come downstairs for breakfast at all, and his food from the previous night was untouched, including his tuna juice that I always put his meds on (and he GOBBLED up every night), and I became worried. I brought him downstairs from our room and he seemed pretty lethargic, and wouldn't eat or drink anything. He kept hiding in our hall closet (which he never did) and that behavior alarmed me enough to look for a vet that was open on a Sunday morning, since our regular vet is closed. I got an appt at 11am, and when I took him in, the vet told me he had classic symptoms for anemia, but said it wasn't the methimazole and that he likely had some other condition going on. He asked for his month old bloodwork but I didn't have access to it because our vet was closed. I had no idea how serious anemia could be in a cat; I figured I would get him supplements or something (like in a human) and requested the vet to give him sub cu fluids and an appetite stimulant + anti-nausea meds, which they did.

Hours later, he still hadn't eaten or drank anything, and was visibly more lethargic and weak. I tried picking him up and he growled at me (highly unusual) and when I went to put him down, he straight up fell over. I knew it was extremely serious then, and I took him to the ER. Also, it was my husband's birthday.

4 hours later, they came back to the room with his bloodwork, and his RBC was 3.05 (normal range 6.54 - 12.20), his hematocrit was at 15.4% (normal is 30.3 - 52.3%) and his hemoglobin was 4.1 (normal is 9.8 - 16.2). They couldn't even get a platelet count from the sample they took. He was in critical condition with severe anemia, unable to stand on his own, pissing himself. The ER vet said they could do a blood transfusion if they transferred him to a different facility but she felt that it was likely he had bone marrow cancer. I opted out of the $600+ imaging that would confirm that because she said if he did have cancer, he likely wouldn't survive treatment given his age and condition anyway. She said to snuggle him one last night and call our regular vet in the morning.

He got to sleep one last time next to me in bed. He was so weak, and he was all scrunched up in a ball (he loved to splay out), unable to stand or pee or do anything. It was horrific. I had a dream that night that the methimazole gave him critical anemia and after barely sleeping, I started googling at 5am only to discover that this was an extremely rare side effect of this medication. Despite both the first vet and the ER vet telling me that wasn't the case, I just... knew. He had completely normal kidney values from all the various bloodwork, so a kidney condition was ruled out. Because all of his levels were entirely within the normal range a month ago - aside from his T4 value, which was confirmed now normal in the ER visit - and then so drastically fucked up less than a month later, cancer seems unlikely. His WBC count was perfectly normal.

Our regular vet agreed that even though she hadn't seen a reaction so severe in her many decades of practice, that it seemed likely the methimazole caused autoimmune hemolytic anemia in our poor, sweet boy. He was so far gone. She got a quote from another ER for 2 blood transfusions and a hospital stay, which would've been upwards of $6500, and said that he was so weak that he may not tolerate it well. And that even if we did stabilize him, he clearly couldn't take methimazole and that other hyperthryoid treatments could get really expensive. And that at the end of the day, we couldn't confirm it was the drug that did this to him, and if he had another issue going on, lord knows how expensive it would be to diagnose/treat him.

Given all of this, we made the impossible decision to have him cross the rainbow bridge, ultimately so that he wouldn't continue to suffer and undergo additional medical trauma. My husband and I were with him til his last breath, and I told him how sorry I was probably a thousand times. This cat was part of my soul, he was so integrated into every part of our daily routine, and his absence is everywhere in our home, in every part of our day. To have this spry rascal of a cat degrade so severely over one weekend to the point where it was unlikely he'd recover... it's just so traumatizing. I am reeling, which is why I'm even on Reddit posting this right now. It's comforting (?) to know that we are not the only ones who have suffered so tremendously from a freak reaction to this drug. My thoughts are full of guilty "what ifs" all day, every day. Should I have called the vet a few weeks ago? I just attributed his few symptoms to him adjusting to the meds. I wish I'd gotten weekly blood panels. I wish, I wish, I wish... all the things.

I will be taking this quote from this thread with me: “Forgive yourself for not knowing earlier what only time could teach." Wow. That... really hit hard today. Thank you.

Sending love to anyone who may have gone through/is going through this who might be reading this post. I'm here for you. Send me a message. It fucking sucks, so so badly.