Thyroid function was studied in 40 patients with chronic heart failure. Thyroid antibodies and microsome antibodies were negative in all cases. Serum T4, and T3 concentrations showed significant inverse correlation with cardiothoracic ratio, mean right atrial pressure, pulmonary artery systolic pressure, and peripheral venous pressure. Serum T4, T3 concentrations showed significant correlation with PaO2, serum albumin, and serum cholinesterase. Serum TSH concentrations increased with increasing cardiothoracic ratio. Histological examinations showed fibrosis and atrophy of the thyroid gland in 2 cases. These findings suggest the possible development of primary hypothyroidism as a result of chronic heart failure.