Radioactive Iodine is the treatment. It is the only way to cure hyperthyroidism. I've never had it done, but I do have a hyperthyroid cat. Beau isn't a candidate for that treatment as he has other health issues. The cat has to be otherwise perfectly healthy to undergo the RI treatment.

Beau takes tapazole in pill form instead. He's supposed to take 5mg every day, but lately he only gets it every second day. He hates being pilled and this schedule seems to be working just as well.

Hyperthyroidism often masks kidney problems. The overactive thyroid makes the kidneys works more. When the thyroid slows down, the kidney difficulties start to show. Unfortunately, that's exactly what happened to Beau. He's in renal failure now. That's why I only give the tapazole every second day. We have to balance his thyroid and his kidneys. The more aggressively one is treated, the worse the other gets.

How old is your kitty? Hyperthyroidism is very common in elderly cats(Beau is at least 15, probably older). It's not a death sentance. It can be managed, even cured in some cats! I hope your kitty doesn't have it, but if she does, you can deal with it!