I was still exhausted 6 months post-RT and my thyroid tested fine like yours did. Had started depression med during RT, Dr thought maybe I needed a higher dose, so increased dose - didn't help. Dr then did bloodwork and found my vit D level was extremely low so I've been on 50000 IU weekly since. It has made all the difference for me. I did 13 weekly doses, then switched to monthly dose, but energy dropped off so dr switched me back to weekly dose. Dr recently weaned me off the depression med - and all is good.

Hopefully the bloodwork will provide some answers for you.