The reason the doctor said you would feel worse for a while after RAI is that you cannot take the antithyroid meds for a while before RAI (they interfere with the process somehow), so you will get more and more hyper. My doctor prescribed a “range” of beta blocker for me to take to provide me with some relief from these symptoms, which worked very well for me. I only had to go to the max dose she suggested for two days, one week after RAI, and then things began to settle down, and I was able to decrease the beta blocker bit by bit. The worst part for me was worrying about it getting worse, and the reality wasn’t nearly as bad as what I had feared. I hope it works the same for you, too.