HI, Shortstuff, I know you will be o.k. You are only 21, such a young person who indeed can live up to 80 if not 100. You know thyroid cancer is a slow growth kind and mostly it spreads to lung or bone and perhaps lymph-nodes if they have been metastassized. Too much radiation surely in not good but it has to be accumulated in a big big amount. I have been going through a lot of radiation, x ray, scan, PET... recently again and agian and I know it may not be good but when thinking the positive against the negative, I still went ahead.
You should not worry about what will happen. Radiation can only lead to a limited type of problem not major cancer and it takes a long long time if it will ever happen and I doubt it will ever occur.. YOu are going to live a long time. Enjoy your life and take your mind out of the cancer shadow. God bless you.