Not dismissing the seriousness of racing hearts but have any of you ever dealt with anxiety? It may also just be that you're working yourself up. Take some deep breaths know that you're fine and it might pass... just a suggestion. Not disagreeing Andrew.
Practice some breathing techniques and after the first few doses I bet your mind will get used to knowing that everything is a ok ; )
HAHA the funny thing is that since thyroid elevation raises catecholamines, anxiety will also increase which could be what is going on with him.