For what its worth, I, too, experienced a sudden energy decrease. 8 weeks after end of rad/chemo, I was able to run several miles, and suddenly even a slow jog was hard work. I had my thyroid checked and found that it was a bit hyper, but a subsequent check found it to be headed back to normal. One of my many docs told me that hypothyroidism, if it is going to show up as a consequence of rad, will be at the 18 mos. to 2 year mark, or later. And yes, my energy did return when I put on a little more weight. Isn't all this post-treatment adjustment fun?