Auricular fibrillation is the most frequent cardiac arrhythmia in hyperthyroidism, and in the paroxysmal form presents itself in varying degrees of frequency and duration. It usually disappears permanently when the hyperthyroidism is relieved.1 In the patients with hyperthyroidism who have auricular fibrillation, although the state of the hyperthyroidism ostensibly may remain unchanged, the auricular fibrillation often is transitory. It seems that there may be some labile factor in these patients which in the presence of the hyperthyroid condition affects the heart so as to favor the production of the transient abnormal rhythm.
Auricular fibrillation may follow the administration of thyroid substances in man.2 This is not of frequent occurrence, however, and the explanation of the mechanism of auricular fibrillation in hyperthyroidism probably is not to be found only in fluctuations in the thyroxine content of the tissues, since this substance is regarded as being too stable to account