Digoxin is a really common drug to treat atrial fibrillation that usually works well, if it doesn't though, there are several alternatives. A-fib is not really related to thyroid problems. The kind of heart flutters from being hyper are not the same. Hope this helps. There should be a cardiac forum to post on to hear if the drug has any side effects.
Here is the link to the Heart Rhythum Community:
http://www.medhelp.org/forums/show/92
and
Heart Disease Community:
http://www.medhelp.org/forums/show/72
I'll let the CL of those communities know you may stop by!
Cheryl