I'm sorry I can't specifically answer your question, but I do know that being hyperthyroid can cause very rapid heart rate, palps, etc and can be dangerous to your heart. I can't tell you exactly what conditions it might cause, but I know there are some on this forum who have been down that road and can answer your question.
Please be patient and I'm sure someone will be along soon.
Do a Google search on "thyroid storm" - you will get all sorts of good information. I've been through it twice and had to be put on beta blockers because my BP and heart rate became dangerously high and I was at risk for stroke/heart attack.
Just went through that myself. I have been on sythroid for 12 years, three years ago I started having palpitations. My doc gave me a tranquilizer and said it was stress. Then in Sept. I had a tachycardia experience, it was really scarey. The emergency room doc said again' stress' since the ekg looked ok. My doc gave me a beta blocker which caused heart attack symptoms . . .the only way to get rid of the angina was to pop three nitroglycerin tabs. All the while I asked every doctor, could it be my thyroid? oh no they said. Then my doc ordered a cat scan of the heart and the report showed an enlarged thyroid . . .so he did a blood test and my thyroid had gone hyper. GEEZ these guys are stupid. So he lowered my dose from 100 to 88 of thyroid and I could feel a difference in two days!!!!! So we will check the levels in six weeks and see where the numbers are. Please get them to check your thyroid, sounds like just lowering the dose might help you out. The arrythmia, tachycardia, and palpitations are really scarey. I am sooooo glad I am still alive. Kathy