I was on synthroid or levoxyl, for over 30 years. My dosage was increased over a long period, trying to alleviate my symptoms until it got to 225 mcg. Because my body does not convert T4 to T3 very well, My T4 levels went up slightly over the top of the range. Shortly after I had some strange feelings in the left chest area, which made me wonder about A fib. Two EKG's later, a stress test, and wearing a Holter monitor for 24 hours, has yet to confirm anything of concern. I still have these feelings that I describe as flutter. My doctor has changed my meds to both T4 and T3, trying to get my free T3 into the upper part of its range, yet keep T4 lower in its range. This has been going on since early Nov. I forgot to mention that this only occurs after I go to bed. Hope this helps.