i can understand your problem. Can you tell me what usually comes in your ECG? There are many possibilities in your case but i would prefer to know the ECG findings. Do you take excess of coffee/tea/any medication for asthma? all of them can increase palpitation
. moreover if indications were not very strong, your doctor may stop thyroxine
for the time being or otherwise he can give another drug which can suppress palpitations. don't worry , since your ECHO is fine. your problem can be managed effectively.
please feel free to write