I too had my thyroid come back several years after RAI! They said it was not probable but was possible. My did not come back as normal size and texture and we did another RAI. It took about 12 weeks before I hit bottom and when I did it was not pretty. One day I woke up and could barely move. Then it was back to synthroid and back on the roller-coaster.
What I was told is that if there is even one cell left after surgery or RAI, there is a chance it can come back. But that chance is so rare that the majority of people will not experience that.
So, at least I know of someone else know that has had the same experience as I did! I see it as a very exclusive club of very extraordinary people.