It probably is not so much a misdiagnosis as a shift in the thyroid itself.
I was diagnosed as extremely hyperthyroid and in a few weeks had gone
hypo.  It was explained to me like this:  I had a virus in my thyroid gland
that made it extremely hyper and after all the excess hormone dumped into
my system (which is when I was really nuts) I then became hypo.  I think it
is pretty common for  hyper glands to become hypo.  At least that is what
my endo told me.  Good luck.

Cindy H.