I had I-131 15 years ago. Other than not bein able to kiss my mate there were no adverse effects. Because the relative dosage is higher for cats, it actually makes them slightly radioactive. That is the reason for the quarantine. I've seen up to 3 weeks to reduce the radiation by 1/8th. The upside of I-131 treatment is that it's a one shot and doesn't require surgery. The downside, at least in humans, as that thyroid replacement may eventually be needed. I'm not sure if cats end up having to take synthroid; I think this treatment is relatively new in cats.

Good luck!