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Subclinical hyperthyroidism is an increasingly recognized entity that is defined as a normal serum free thyroxine and free triiodothyronine levels with a thyroid-stimulating hormone level suppressed below the normal range and usually undetectable. It has been reported that subclinical hyperthyroidism is not associated with CHD or mortality from cardiovascular causes but increased factor X activity in patients with subclinical hyperthyroidism represents a potential hypercoagulable state. It has been also reported an acute myocardial infarction with normal coronary arteries associated with iatrogenic hyperthyroidism and with a myocardial bridge too. Moreover, it has been reported that simply measuring maximum P wave duration and P wave dispersion values, may help to determine the patients with subclinical hyperthyroidism and high risk for the development of atrial fibrillation. We present a case of an acute myocardial infarction without significant coronary stenoses associated with subclinical hyperthyroidism. Also this case focuses attention on the importance of a correct evaluation of subclinical hyperthyroidism.