Anaplastic thyroid carcinomas are very aggressive cancers and metastasize frequently to the lungs and pleura. The cardiac metastases of thyroid cancers are rare. The diagnosis before the death is rare. We present a case of anaplastic thyroid carcinoma with metastatic involvement of the heart. It was presented with atrial fibrillation. The echocardiography demonstrated a solid mass in interventricular septum. One week later the patient died of heart failure. We call attention of clinicians that during the follow up of thyroid cancer such arrythmias may be the harbinger of a cardiac metastasis.