So you think her overdose is responsible for the high amylase and BUN?
Amylase is not reliable in cats, so the elevation of that enzyme has no significance. BUN is a different matter. It is one of the kidney values, along with creatinine. Creatinine is kidney-specific, but BUN can elevate for non-kidney reasons. The fact that both your cat's BUN and creat have risen, however, does imply some degree of renal insufficiency, even though her creat is still in the high normal range.
The methimazole is probably not responsible for the elevation in her kidney values. It's more likely that it just unmasked pre-existing renal insufficiency. Uncontrolled hyperT causes all of the body systems to work in overdirve, including the kidneys. If the kidneys are diseased, as is likely the case with your girl, hyperT's influence will make them work harder, and they may appear normal in bloodwork as a result. That's what may have been happening at the time of your girl's first blood test. Her uncontrolled hyperT at that time was making her diseased kidneys work harder, masking their true condition in her bloodwork. Now that the hyperT has been turned to hypoT and her metabolism has slowed waaaay down, the true condition of her kidneys has become apparent in bloodwork through the elevations in her kidney values.
The vet said he thought the elevated BUN might be due to her still being a little dehydrated. Would an overdose on methimazole cause her to be dehydrated?
Her bloodwork doesn't give the impression of her still being dehydrated, so I don't believe that's a factor. As far as I know, methimazole overdose would only cause dehydration if it also caused vomiting and/or diarrhea in the cat.
I think she felt a bit itchy for the first day or two. She was lethargic before taking methimazole and she still is but I've seen improvement. She still needs to regain muscle mass but she has made a few jumps she was to weak to make before. It also became harder to make her pur and she became more aloof.
It's difficult to tell which of her symptoms and behavior are or were attributable to the hyperT, hypoT, high methimazole dose, renal insufficiency, dehydration, and/or emaciation.
Her appetite had been decreasing, I guess the low T4 could account for that though.
Yes, it certainly could, but so could any existing renal insufficiency and/or nausea caused by it or the methimazole. Still, the fact that she's gained more than a pound in a month is a very good indication that she's eating plenty right now, and that's great!
Currently, I give 1.5 oz of wet food twice a day. Since she has been effectively eating all wet food as a result, I should probably just offer her more right? My gut feeling is that she would eat one and a half cans a day, maybe two.
I would eliminate all kibble from her diet (esp. with questionable kidney health) and feed her 5-6 oz of canned food a day split into 3-4 small meals.
It would also benefit her to mix some warm water into each meal to make it a thick gruel that she can lap up, if she'll eat it that way (most cats love canned food gruel). That would help keep her properly hydrated which will help support her kidneys.
You said in your first post that she had always been skinny at 8 lbs, so I'm guessing that her ideal weight would be somewhere between 8.5-9 lbs. With her hyperT controlled, you will hopefully be able to bring her up to that weight over the next few months.
BTW, your vet is now having you give her 1.25 mg of methimazole twice daily, right (I hope)? Not 2.5 mg once daily. She will maintain more even thyroid hormone levels if she is dosed twice daily.