I never treat hyperthyroid cats with methimazole or with I3 (radiation). What I’ve always done is surgically remove the thyroid tumor. It results in immediate resolution of the problem and doesn’t require a lifetime of a weird drug that’s hard to balance and regulate. The surgery is very superficial for the kitty so not very traumatic (little more than a small incision in the skin). It’s a surgery that requires some skill and finesse from the surgeon but is no bid deal to the cat. During the surgery, you have to isolate and remove the parathyroid gland from the thyroid and poke it back into the kitty (cats like having a parathyroid gland) but other than that it’s a simple affair. Make sure the vet doing it has experience doing it, but other than that, no big deal for kitty.