RAIU uses a different type of
radioactive iodine. It is not designed to kill
thyroid cells and is reasonably safe to do the uptake, although I don't think you would want to have one done every week.
RAI 131 is used to kill thyroid tissue.
RAI 121 is used for uptakes scans, I think, although that may be wrong. At any rate, it is two different forms of radioactive iodine.