Thyroid hormone and heart failure.
Thyroid hormone metabolic disarray has been identified as a risk factor for the progression of heart disease and the development of heart failure (HF). Both hyper- and hypothyroidism have been associated with a failing myocardium. Poor cardiac contractility and low cardiac output due to hyperthyroidism is a rare occurrence and is mostly seen in patients with preexisting heart disease. Referred to as a "rate related" phenomenon, hyperthyroid-induced sustained sinus tachycardia or atrial fibrillation may further reduce ventricular contractility. Increasingly, the hypothyroid state, and in particular a low triiodothyronine level, has been associated with a reduced cardiac performance and poor prognosis in HF, even in the presence of normal thyroid-stimulating hormone levels. Low thyroid hormone levels alter cardiac gene expression and increase systemic vascular resistance, both resulting in a reduction of cardiac contractility and cardiac output. This review summarizes current data on thyroid dysfunction and HF as well as the emerging implications of the "low triiodothyronine state."