There are so many different causes of bradycardia. It can even be caused by a low thyroid function which is what my roommate has. She has hypothyroidism. Apparently, thyroid hormones control many different things in the body such as metabolism, breathing rate and heart rate. So very low levels of thyroid hormones can cause a slow heart rate. My roommate is fine when she takes her thyroid medications. But when she doesn't, she becomes very tired and cannot do anything.