Yes, definitely have your thyroid levels checked. Also if this has been going on awhile, you need to talk to your Dr about wearing a 24 hr monitor if you haven't already. You might even need to go on a blood-thinner if you are in a-fib for longer than 24 hrs at a time. With a-fib you can be at greater risk for stroke. Caffeine, alcohol, cold meds, all kinds of things can trigger these episodes. This is a progressive disorder, in other words it won't just go away on it's own. Meds can help for a time but an ablation can often put an end to it.
Sorry to hear you're having such a hard time. Have you seen an electophysiologist who specializes in the electrical system of the heart?Another thought might be to talk to your doctor who is managing your thyroid med. An out of whack thyroid can really mess with heart rythm in some people. Good luck. I hope you feel better.