I've been experiencing something similar to this for a couple of years now. In my case, it is like I only have so much of "something", and when this "something" is depleted, or its level goes down considerably, I'll start getting sore muscles, brain fog, and then the chills after an intensive workout or physical activity. You write, "I have not been able to work out enough to get a sore muscle..." and I'm the same; my muscles will get sore but it isn't the sore as years previous to this when I worked out, its just an aching of the body, and it isn't because I worked out too much. I can't work out too much because this will happen to me. So I too have a hard time keeping in shape. I too have always been an athlete, but then this happened and I can't keep to the level of fitness that I want. After going down so many pathways, looking for a solution to this, I've put this down to it being my thyroid and I needed to get my TSH level much lower. My lab tests have just showed that I'm in the upper level of what is "normal", to the point that my doctor thought I might not even have a thyroid condition. So to find out, I went off it. Four days later, after a yoga workout, I was in bed freezing to death, badly. Went back on the medication and I'm better. I could do my workouts, but not two days in a row, needed to give the body a break in between. And then we increased the amount of medication. Now I can workout multiple days in a row without symptoms. But if I really overdo it I still get minor symptoms. So I'm thinking I could still increase it a little more and considering doing so. Bottom line: Some of us can show that our TSH is only just slightly elevated (T3 and T4 are fine), still show within the normal range, but still get severe symptoms like I describe above. Increasing the dosage worked for me.