Methimazole and anemia

Good Afternoon, I hope this finds you well. My (now 15 year old approx.) female spayed cat was diagnosed with hyperthyroidism in 2020 and given oral methimazole/felimazole tablets which we tried but she would not take them and when she did was vomiting a lot. So (much to my regret now!) I felt lost and didn't do anything about it until March 21 when she had lost more weight (I thought she'd have the same reaction to all methimazole because of the drug reaction rather than it being the method of delivery) so I went to get her levels checked again, they had gone up and I asked if I could try the transdermal gel instead. This was fine! There was no vomiting etc and she seemed to be okay,  but not putting any weight on. I got her tested in June to see how her levels were but they ONLY CHECKED T4 and didn't do a full blood panel which I now understand is imperative after starting the medication. I worried at her continued weight loss and was concerned about CKD or kidney function (she was drinking a lot still) so had her bloods done again and it showed her as being anemic. I started giving her b12 tablets to see how she did and continued with the methimazole. Took her back last week for another check and her anemia is worse and vet can feel inflammation or hardness in either her small intestine or spleen. They suspect a slow bleed around that area (she does bowel movements outside) so can't really check for blood in stool etc if it would even show up (or the vet also mentioned in could be a mass :( ) My question is, could it be the methimazole causing the anemia as I've read this is a fairly well documented possible side effect of methimazole (though fairly rare) and could it be the anemia causing the swollen spleen? (I've found some info that had said anemia over a long period can present this way) I bet vets hate pet parents googling stuff 🤣 I'm also not sure if I'm just in denial or desperate about her prognosis but I just really have a sneaking suspicion about this being the case, especially because there wasn't a full blood panel done when it should have been. Any input would be much appreciated! Thanks, Kind regards, Gemma  

Hello!  Did you know about data sheets?  These are sheets of impossibly dense text that are folded down very small and should be included with your tablets.  Most people thrown them away with the packaging to be honest, although it isn't advised.  They contain all the 'rules' and facts for that medication.  If you don't have the sheet, don't worry - your vet will have a book of them for every med.  They include all of the known side effects and doses for licensed medications. There should be a sheet specific to the tablets you are giving and you will probably find anaemia on it. The medication that you mention can cause anaemia and may be contraindicated in anaemia  (ie you may need to stop giving it when anaemia is seen!) So we would suggest running this by your vet or emergency vet as soon as possible.  You don't mention how anaemic the patient is or whether supportive treatment was required, or even how you know there is anaemia (beware that some blood machines are made for reading human bloods habitually record low numbers in cats so the vet may be testing the PCV manually, another way and finding it to be normal - important to ask them).

Indeed, anaemia can cause swollen spleens (low red blood cells cause a contraction to produce more) and swollen spleens (eg cancer, I'm afraid may be on the list) can cause anaemia.   I would advise seeking further help and medical assistance with this case.

It may be that your vet already knows of this conraindication and has a reason not to follow it; the first point of call would always be to ask them.