When I ccross check them for interactions in ePocrates, a drug database put together by doctors for doctors, "No significant interactions found or known."
I have never heard or read of any interactions. One thing to keep in mind is that until your thyroid levels are optimized, you are still hypo, and being hypo your bod runs slower and drugs may act longer.
I am not a doctor, nor have I ever played one on TV...
I took synthroid and xanax for a while. It worked out OK. I do agree with the others that you should talk to your pharmacist.
My thyriod is still a little out of whack (waiting for the new dose of synthroid to kick in). I did find that once my first dose of synthroid had kicked in, the symptoms I was having that promted my doctor to prescribe the xanax had subsided. I don't know if that will be the case for you. But I was able to stop the xanax after a while.
"Life, for all its agonies...is exciting and beautiful, amusing and artful and endearing... and whatever is to come after it-we shall not have this life again."