Hypothyroidism is a rare cause of dilated cardiomyopathy. Triiodothyronine was found to target endothelial nitric oxide synthase, which serves to enhance peripheral vascular relaxation by acting on vascular smooth muscle cells. In patients with subclinical hypothyroidism, adenosine-induced coronary flow reserve is significantly reduced. We report the case of a 46-year-old woman with hypothyroid cardiomyopathy manifested as reversible severe left ventricular dysfunction (LVEF = 32%). 15O-labeled water PET at rest and during cold pressor test showed a severe coronary endothelial dysfunction. In patients with severe hypothyroidism, endothelial function is impaired and may be an important mechanism of heart failure.