Inappropriate secretion of TSH (IST) refers to a heterogeneous group of syndromes in which patients show unsuppressed TSH levels in spite of high serum free thyroid hormone concentrations. It has been recognised that IST can be due to both thyroid hormone resistance (RTH) and pituitary TSH-secreting tumours. The former can be generalised (GRTH) or pituitary (PRTH) if the resistance is more severe in the pituitary than in the remaining tissues. This case report describes a peculiar coexistence of atrial fibrillation and mitral valve prolapse in a patient affected by generalized resistance to thyroid hormone. This finding is suggestive for a major and almost physiological sensitivity of the myocardium to the thyroid hormones activity which in the course of years may determine the modifications responsible for the pathologies described.