Do I really need to stay away from my cat for 2 weeks following iodine treatment for hyperthyroid? Is this just being overcautious?

Yes, it is probably very over cautious if the cat was at the vet for 5 days or so, unless you are a child or pregnant, BUT it is only for two short weeks, so why chance it. Follow your vet's advice on quarantining your cat, and disposing of the litter. For such a small period of time, you can afford to be safe and NOT sorry. Follow their advice about aftercare and handling litter. You can still have some limited contact and petting, unless you are a kid or with-child, but don't let your cat sleep on your bed, or roam where you cook your meals. Two weeks will fly by. I hope your kitty is feeling better soon.

It isn't over cautious at all - my sister had this treatment and the staff explained all the dangers of radiation overdose to her. Even a small dose is very harmful to a cat or child due to their small body size, I know its hard but you really need to keep your pet away until you are given the all clear on the radiation