Two patients, both 70 years old, operated for differentiated thyroid cancer with foci of undifferentiated tumor (one papillary containing pseudosarcomatoid foci, the other with insular pattern containing undifferentiated foci) suddenly died. In both cases, for both of them metastatic involvement of the myocardium and endocardium. These original findings suggest that elderly patients suffering rom well differentiated thyroid cancer, containing undifferentiated foci, therefore suspected to have metastasized, would benefit from non-invasive cardiac work-up (sonogram) in order to plan their multidisciplinary treatment.