Occasionally heart palpitations can be a sign of a serious problem, such as an overactive thyroid gland (hyperthyroidism) or an abnormal heart rhythm (arrhythmia). Heart rhythm changes (arrhythmias) might cause a very fast heart rate (tachycardia), an unusually slow heart rate (bradycardia), a normal heart rate that varies from the usual heart …
https://www.mayoclinic.org/diseases-conditions/heart-palpitations/symptoms-causes/syc-20373196

