Experimental and clinical findings strongly support the concept that thyroid hormone (TU) has a fundamental role in the cardiovascular homeostasis both in physiological and pathological conditions. In heart failure (HF) the main alteration of the thyroid function is referred to as low-T3 syndrome characterized by the reduction in serum total T3 and free T3 with normal levels of thyroxine and thyrotropin. This syndrome, that affects one third of HF patients, is considered as adaptive factor minimizing catabolic phenomena of illness. However this interpretative hypothesis is actually questioned. In fact experimental data showed the potential effects of this syndrome in the progressive deterioration of cardiac function and myocardial remodeling of HF. Prognostic studies have shown that T3 levels represent a powerful predictor of mortality in HF patients, also adding prognostic power to conventional cardiac parameters. Large, multicenter, placebo controlled prospective studies will provide the safety and prognostic effects of the chronic treatment with thyroid hormones in HF.