I'm sorry your kitty isn't feeling well. I went through this with my cat, Sophie, last December. We opted for the radioactive thyroid treatment. Minus missing her for a few days and then not being able to pet her much when she came home, it was a painless procedure.
Talk to your vet, if you haven't already, about your cat's kidneys. Thyroid disease can mask kidney problems and also sort of keep them in check because of the increased blood volume that it causes. If your cat's kidneys are already failing, then pills or a transdermal application might be the better option. With careful monitoring, the vet can find a balance in treating both diseases.
Good luck with your kitty. I hope she feels better soon.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher