My Sophie was diagnosed with HyperT at age 12 (we lost her to cancer this past Oct. at age 17). She was on methimazole for 5 years, and it made all the difference in the world. She went from being an insatiable eating-machine with coarse, brittle hair to her usual beautiful tuxedo-cat self with a healthy appetite. I did have to watch her eating habits over the years, and adjust her meds accordingly. Her coat was also a good indicator of how her meds were working.
It sounds as if you're being a very good cat-Mom. Best of luck with Banzo.