Depending on its severity, hyperthyroidism can result in a variety of complications, including atrial fibrillation. Atrial fibrillation is an arrhythmia condition where the heart's chambers quiver ineffectively rather than beating. Without effective beating, blood pools in the chambers and can form dangerous clots. Fifteen percent of all strokes occur due to blood clots from atrial fibrillation. If you have hyperthyroidism, it is important to carefully follow your treatment plan and be aware of the warning signs for stroke and heart attack. With effective treatment, hyperthyroidism can be managed and dangerous complications, like arrhythmia, can be avoided.