D.M.
Hi C.! I also had my thyroid (and various other parts) out this summer due to metastatic thyroid cancer. It sounds like your synthroid might be a bit high. Often, after the surgery, the patient is put on a slightly higher, theraputic dose to get the TH (thyroid hormone) level back up to where it would be naturally and it might just be that your levels have stabilized and you just have too much.
I had lots of symptoms from the cancer, but also had a couple of othr thyroid issues going on at the same time (I was a MESS!) and had a high heart rate with arrythmias. My doctor gave me a low dose of a beta blocker that really helped until, in my case, my surgery. Good luck!