I'm not a vet but this Felimazole sounds like a wicked drug. Felimazole - FDA prescribing information, side effects and uses

What it looks like to me is some sort of lesion, and if you read the adverse effects, that is one of the things that can happen with this drug. Did the lesion appear before or after your cat was on Felimazole? I would contact the vet who prescribed this. A second opinion from another vet may be useful.

 

It looks to me like it's possibly feline acne. Fell his whole chin gently, I use my fingernail, to see if there's any other bumps. IME feline acne can start to look red and irritated, or like an open sore, if they scratch it too much.

I wouldn't bet on it being the fountain, as long as you clean it regularly (once or twice a month) it shouldn't be doing too bad on bacteria. Assuming it has a filter.

A bigger cause would be not cleaning the bowls every day - it's great if their stainless, but they have to be cleaned every day for that to matter. Ditto for ceramic.

You could try wiping her chin twice a day with Listerine on a cotton pad. That works the best (next to the medicated stuff from the vet) on my guys when they get it. As long as you stay on top of it acne isn't too big of an issue.

 

Oh, trust me, I am NOT happy with the Felimazole - I looked at its side effects when the vet first prescribed it and I just about had a fit. Plus Win is a known kidney disease cat and the Felimazole absolutely made her kidney values worse. I know treating the hyperT is more important, but it still bothers me.

What I WANT is the radioactive-iodine treatment (I-131) which is an instant cure for hyperthyroidism, but my vet seemed pretty upset that I even suggested I-131 and was completely against it. Granted, Win is 17 and does have renal disease, but from what I've read, a competent vet who has done I-131 before knows how to handle the treatment in elderly cats with renal disease. Felimazole is NOT a cure for hyperT and Win would have to be on it for the rest of her life, probably on increasing dosages as the thyroid tumor got bigger. If my vet is still completely against I-131 after this, I might have to find a new vet.

As an aside, since Win has been on the Felimazole, she's become lethargic, listless, and has no appetite. So she's definitely already having some side effects I'm not happy with, and if this is a skin lesion, I'm done with the Felimazole.

Librarychick, I don't feel any other bumps on her chin, which is why I started to worry this wasn't feline acne. I've been putting Zymox on the spot too and cleaning it twice a day, and it hasn't done anything. :{

I do clean her bowls after every meal (she eats 3 times a day in small portions). I have 3 bowls that I rotate. I also change her water and clean the ceramic waterbowls daily and sometimes twice a day if I notice she's gotten a lot of fur into the bowls ;}

I HAVE had these stainless steel bowls for a few years now, though, so I'm going to be replacing them with new versions of the same type, just in case.

I have a vet appointment for tomorrow morning (with a close-by vet; I can't get a ride to her normal vet) so we'll see how that goes. I asked a JustAnswer vet and showed them the pictures and they scared me by saying it was possibly some kind of skin cancer and I should get it checked/removed early, so off to the vet we go!