Of course, if the pounding and rapid heart rate become too troublesome, don't hesitate to call your doctor or go to the ER.
The risk of heart
palpitations and heart attack
, don't come, simply, from taking the medication
, itself; they come from being over or under medicated. The only way you can know, is to be testing Free T3 and Free T4, at the same time TSH is tested, and just being in a "normal" range is often not good enough. You have to be a level that's right for you.