hyperthyroid cat- continuing weight loss- ideas?

My little female cat is probably around 12 or 13 yo. She has always
been a small 6 pound cat. In early December at her annual
vaccination, she was down to 5 pounds 3 ounces, I had a blood panel
done. T4 was 16, BUN and creatine were normal. She was started on
2.5 mg Tapazole twice per day. I had to leave town for 2 weeks near
Christmas and boarded her at a good facility. By then, her weight was
up a to about 5 pounds 8 ounces so things seemed to be going well.

During boarding, everything went wrong. She wouldn't eat, then they
found food she liked- but even so her weight declined sharply, by the
time I picked her up she was down to 4 pounds 1 ounce. The vet did a
urinalysis, it showed infection, so now she is on twice daily
amoxycillin as well as the Tapazole. The specific gravity was 1.025,
indicating probable kidney problems.

Now she is at home. She eats ravenously. I open cans of her favorite
food 5 or 6 times a day and each time she eats with great enthusiasm.
Her behavior and demeanor are completely normal. But she is horribly
thin and seems unstable when jumping from one level to another. My
plan is to feed her as much food as she will eat for a week, try to
fatten her up and get her blood levels checked again.

What really worries me is that perhaps treating the hyperthyroid
condition is unmasking a more serious kidney condition. But this
boarding disaster may simply be due to stress (although she has
boarded with no trouble quite often before) or infection (she has also
been sneezing since getting home). Any ideas or experiences other
people have had would be appreciated. Thank you.