REN and Myocardial fibrosis: Considering that: 1) myocardial fibrosis depends on a complex relationship between stimulatory (including: angiotensin II, endothelin I, catecholamines and aldosterone) and inhibitory factors acting on the collagen production [22]; 2) renin-angiotensin-aldosterone system is activacted in patients with hyperthyroidism [13,14]; 3) thyroid hormones may influence the responsiveness and sensitivity of cardiac tissue to normal sympathetic stimulation [13]; it is reasonable to admit that these mechanisms could mediate the increased myocardial texture observed in these patients.