Hi, Higher thyroxine levels as well as rigorous exercises can make your heart rate to beat harder & faster, which sometimes causes disorganized heart rhythms and some times missing of some beat. In your case, if the thyroid profile has been tested long back, you should get it tested, as your dose of thyroxine may need adjustment. Similarly in your case, the exercise should not be rigorous. The level of exercise may be increased gradually depending on feeling of missing heart beat or palpitation etc. In the beginning just walking on a brisk pace, swimming or stationary cycling may be part of your routine exercises. Still, one thing must be kept in mind that missing a few beats per minute may be very much normal. But if it is frequent occurrence or causing palpitation, dyspnea, dizziness etc., then it is an indication to see your doctor. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Satnarain Bansal, General & Family Physician