It still could be your thyroid -- your endo could be wrong or not thorough. This is common with endos, and it's not easy getting substitutes to work as well as your natural body did. Only way to tell is to see a different endo and get a second opinion. Now, let's assume you've suddenly developed a panic disorder just for kicks -- in that case, the effects can last a long time or a short time depending on the person and the trigger. People are different. But if this is the first time you've ever had one and the feelings are lasting this long but you don't feel anxious, then again, get back to an endo and insist that you don't feel it's adjusted properly. Now, I knew this before, but my wife got Hashimoto's and I've lived through the ups and downs of Synthroid with her and the ups and downs of trying to get her endo to listen to her.