It’s not such a big deal, promise. 
My Fred cat is 18, been on the methimazole pills for over a year now. He’s on the highest dose my vet feels comfortable with in order to keep it in check, but seems happy and content. He takes his pills twice a day like a good egg. He’s also got some sort of weird neurological thing that popped up in the last 6 months, which he gets prednisone for.
We decided against the idodine treatment. My vet quoted me about $3000, all said and done, and even though that’s a ton of money I would have considered it except that he HATES being at the vet and is a complete homebody, so I figured at 18 years old it was better to keep him as happy as possible.
The most important thing is to make sure that they keep eating, and keep their weight up. My guy has dry available 24/7, and I give him a spoonful of wet food everytime he asks for it. (Which, since I think he’s slightly senile, is about ten times a day. :lol: I just keep a can covered on the counter and pop a spoonful in his dish.) On a good day, which is most days, he’ll go through two regular cans of cat food. On a bad day, it may just be a single tiny can of Fancy Feast. He is picky picky picky so I regularly bring home different brands and varieties for him to try.
Everyone in my household eats grain free, but he is an old man set in his ways and turns his nose up at any of the grain free I’ve ever put in front of him. He likes the junky Friskies and Fancy Feast, and at this point in his life, I’d rather eat than have a battle of wills with him and watch him waste away.
I also make sure he has fresh water in front of him all the time; I rinse and refill his little bowl almost every time he gets the wet food. He’s creaky and old anyway, so I make sure that everything is in easy reach for him; your mileage may vary if your cat is still younger and frisky!
Deep breaths…it’s not a death sentence, and my vet told me that the iodine is basically a cure, so that’s fantastic that it’s an option for your kitty!