An arrhythmia is a heartbeat that’s not regular. Examples would be beating too slowly (bradycardia) or too quickly (tachycardia), which causes problems like dizziness or fatigue. In most cases, arrhythmias are caused by an electrical “short circuit” in the heart, but they can also be caused by hormones produced by the thyroid. Many children with a heart arrhythmia never experience symptoms and don’t need medical care. However, in some cases, an experienced pediatric heart expert will need to identify and treat your child’s arrhythmia.