Here's a really good link on I-131 treatment. http://thyrocat.com/inside/radioactive-iodine.html

One of the volunteers in my rescue group had this done to one of her cats. Younger cats benefit more from this, as it can be expensive. The biggest pain is keeping the radioactive litter for 3 months, but my friend's cat responded very well to the treatment. (Also, the cat was very difficult to pill.)