(1) Enlargement of the heart was found in 35% of a group of older patients with thyrotoxicosis who had no evidence of associated heart disease. Atrial fibrillation was present in 48% of patients with cardiac enlargement and in 10% with a normal sized heart. Both complications were more common when there was associated heart disease.
(2) In patients with an enlarged heart but no associated heart disease a reduction in heart-size after treatment of thyrotoxicosis was observed in five of twenty-three who had sinus rhythm throughout, and in three of eleven who had atrial fibrillation on presentation with subsequent reversion to sinus rhythm. There was no reduction in heart size in ten who had persistent atrial fibrillation after completion of treatment. Reduction in heart size was most marked in patients who had presented with frank cardiac failure.
(3) The incidence of cardiac enlargement and atrial fibrillation both increased with advancing age but whereas enlargement was common over 45 years atrial fibrillation was rarely seen under 55 years of age. Only one of thirteen patients with an enlarged heart under 55 years had atrial fibrillation.
(4) It is concluded that enlargement of the heart may be a manifestation of thyrotoxic heart disease, that it may precede the onset of atrial fibrillation in some instances, and consequently that atrial fibrillation is not necessarily the cause of an enlarged heart in thyrotoxicosis.
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