Although the normal heart rate in adults is 60-100 beats/min, a heart rate of about 55 beats/min due to physiologic causes is usually not dangerous, and may be dangerous if it is due to pathological causes, such as hypothyroidism. 1. Physiological causes: (1) This situation is common in the physical fitness of people or athletes, they may be in a quiet state heart rate of about 55 beats / min, if not accompanied by dizziness, palpitations and other uncomfortable symptoms, generally do not need to worry. (2) If the heart rate is slower than that of the daytime due to the vagus nerve excitation in some people during the nighttime sleep, the heart rate will be 55 beats/minute, at this time the blood supply of the heart can still satisfy the needs of the whole body, and there is no need to worry about it in general. 2. Pathological causes: For example, in hypothyroidism, the patient’s own secretion of thyroid hormone is reduced, and the effect of thyroid hormone is weakened. As a result, the heart rate slows down, while the cardiac output decreases, etc., which may cause heart disease. When an abnormal heart rate occurs, it is recommended to consult a doctor in a timely manner, follow the doctor’s instructions for further examination, clarify the cause with the help of the doctor, and carry out targeted treatment or therapy.