NPPB and thyrotoxicosis: Furthermore, our observation of lower NT-proBNP levels in RTHβ than in newly diagnosed, conventional thyrotoxicosis suggests that hyperthyroxinemia of lesser magnitude but of longer (lifelong) duration in RTHβ leads to myocardial dysfunction, including nonischemic cardiomyopathic changes found on echocardiography [13, 16], which can manifest at a relatively early age in this disorder.