Definetly have the full blood panel done. It will show both the thyroid AND the kidney function (which is from the BUN level). The IBD **could**be causing the problem, but, I bet it is the thyroid combined with the kidney. What was the 'upper' level of the thryoid? Since it is so easily controlled with meds (tapizole), I would suggest to your vet you try the lower dose, one time a day. That should be 5 mg. I have seen a low dose tapizole work wonders. The kidney function is controlled with calcitriol. A liquid, mixed by a compounding pharmacy, not a typical pharmacy pharmacy. How do I know? This is EXACTLY the symptoms Tex presents. Also, I now do a quarter tab of pepcid AC...twice a day. The tummy develops ulcers sometimes, that work to inhibit ingestion, and cause the kitty not to have an appetite. What happens is that a very vicious cycle develops. The kitty has a slight kidney malfunction, loss of appetite, which compounds the kidney process, which makes the malfunction worse.....

Also, with the utmost respect to your vet....kidney function IS determined with blood...so, please re-examine the blood panel. Tex gets a full panel twice a year, and has for 3 years now.

One more thing on the RX food...just cause it is RX doesn't mean it necessarily is low in protein. Some is, some is just termed 'sensitive'. KD and ZD are both rx foods, low in protein. Protein for cats, while a necessary dietary need, stresses the kidney. I have battled Tex with his desire to eat nothing but chicken with the increase in BUN levels. I actually just gave up on the rx food, cause he hated it, and was losing weight. I switched to plain old fancy feast. He eats it, it has a higher water content that dry food. You have to make a decision sometimes with how to best accomplish the medical care with the kitty's best interest.

I am happy to tell you more..but, an accurate blood screen is first. Good luck.
Johanna