Hello - I posted a similar situation a few months back as well. I had a partial thyroidectomy. I experienced extreme fatigue to the point I couldn't get out of bed and bad muscle pain. I also am experiencing weight gain.
My first blood work showed I was now hypothyroid (levels were always good prior to finding out I had cancer.) The doctor said I would feel much better after getting my levels up with Synthroid. He put me on 75 mcgs. My last labs showed my thyroid levels were in optimum range and I was very surprised since I am still experiencing the symptoms. Its been six months for me. Follow up ultrasound next week. I don't understand what is happening and I am sorry to hear that you are experiencing the same. I am constantly searching for answers and will be sure to share should I find out something that will help make sense of it all. Best off luck to you and take care!