Atrial fibrillation (AF) is a condition of irregular heartbeat. While AF can present with no symptoms other than an occasional elevated heart rate, some people experience dizziness, a feeling of a racing heartbeat, palpitations, weakness, confusion, shortness of breath, fatigue, inability to exercise and lightheadedness.
AF may be occasional or paroxysmal with symptoms that come and go, lasting a few minutes to a few hours before resolving. AF may also be persistent, meaning it doesn't resolve without treatment (medications or electric shock). In long-standing persistent AF, the condition occurs for longer than 12 months. In permanent AF, the abnormal heart rhythm can't be restored and requires life-long medication.
AF Causes
There are a number of causes of atrial fibrillation. AF occurs when the two upper chambers of the heart experience chaotic electrical signals. This causes the upper chambers to quiver and the atrioventricular node (connecting pathway between upper and lower chambers) is bombarded with signals that are trying to reach the lower chambers (ventricles).
While AF can occur in hyperthyroidism, this represents only about 10 percent of cases. Other causes include hypertension, coronary artery disease, heart attacks, abnormal heart valves, congenital heart defects, use of stimulants, sick sinus syndrome, pulmonary disease, previous heart surgery, viral infections, sleep apnea, and stress related to other diseases such as pneumonia. In pneumonia, the body's need for thyroid hormone decreases and even slightly elevated thyroid hormone levels can trigger episodes of thyroid storm and AF. In people with lone AF, there are no proven causes and the consequences are rarely serious. Family history may play a role as AF tends to run in some families.
Autoantibodies in Graves' Disease
Research shows that Graves' patients with AF have activating autoantibodies to M2 muscarinic receptors (AAM2R) and beta-1 adrenergic receptors (AAbeta1AR). Stimulation of adrenergic receptors causes symptoms such as anxiety and elevated heart rate that are commonly seen in untreated hyperthyroidism. Drugs known as beta adrenergic blocking agents or beta blockers prevent the reaction with the adrenergic receptor and reduce these symptoms.
Previous studies have shown these 2 autoantibodies in some patients with dilated cardiomyopathy and AF. For this reason, researchers conducted a study on Graves' disease patients and found that patients with AF have these antibodies, which are immunologically distinct from TSH receptor antibodies. The authors concluded that when present in Graves' hyperthyroidism, AAbeta1AR and AAM2R facilitate the development of AF. Further studies are needed to show if these antibodies are also linked to first degree relatives with atrial fibrillation of patients with Graves' disease.
Resources:
Atrial Fibrillation, The Mayo Clinic. 2015. http://www.mayoclinic.org/diseases-conditions/atrial-fibrillation/symptoms-causes/dxc-20164936
Stavrakis S, Yu X, Patterson E., et al., "Activating autoantibodies the the beta-1 adrenergic and m2 muscarinic receptors facilitate atrial fibrillation in patients with Graves' hyperthyroidism.: American Journal of Cardiology, Sept 2009; 54(14):1309-16.