That is a very, very rare condition that you have, but it causes Synthroid to be at periodic overdose levels because of the swing of levels in your system. Overdose of any thyroid replacement therapy would cause those problems.
Moral of story, never trust any doctor but an endocrinologist with metabolism and especially thyroid problems. After I was diagnosed I asked my regular doctor about hashitoxicosis, he said he had never heard of it. I told my fabulous endo guy what he said and he exploded, said tell him to go back to medical school, that he caused me to suffer unnecessarily for years.