I'll have to check back for more answers to your question as I have a cat that is going back today for a freeT4 test since the T3 and T4 came back odd. Hyperthyroid on a cat is a new walk in the park for me and all I know is there is low iodine diets and medication. I wasn't aware of this treatment as well.
S.
EDIT: I hope you have been able to find a solution for your cat. Mine is indeed hyperthyroid and we have started her on 2.5mg of methimazole crushed added to canned food. The radioactive treatment is costly where we live and the other option was Science diet y/d and at 2.10 a can and only out for less than a year, the medication seemed to be the way to go.