Beginning in development and extending to adult physiology, a close relationship exists between the thyroid gland and the heart. They migrate together during ontogeny and possess common factors of transcriptions. Among them, Nkx2-5 whose mutations were recently described in patients with congenital hypothyroidism. The genomic impact of triiodothyronine (T3) on cardiac tissue allows the activation of synthesis of proteins belonging to the contractile device. The progress realized in the understanding of the mechanisms of cardiac remodelling strengthens the links between both organs : for example the recent discovery of interactions between some microARNs, activated in case of heart failure and the thyroid hormone receptor. There are numerous parallels between hypothyroidism and heart failure. The low- T3 syndrome are the single most significant predictor of cardiovascular and all-cause mortality in adults with heart disease and the treatment by T3 in these circumstances could allow improvement in cardiac function.