Hürthle cell carcinoma (HCC) is a variant of a follicular carcinoma with a tendency to higher frequency of metastases and a lower survival rate. However, intracavitary cardiac metastases from thyroid HCC are extremely rare. We describe the case of a 57-year-old female with thyroid HCC, 5 years after total thyroidectomy, who presented with dyspnea associated with hypoxia and hypotension. The computed tomography angiogram showed extensive pulmonary embolism and a 6-cm right atrial mass while the lower-extremity deep vein thrombosis studies were negative. This patient received a cardiac thrombectomy using cardiopulmonary bypass support. However, intraoperatively, we found out that the mass was from the mediastinum, directly extending into the heart and clearly unresectable since it effaced at least 1/3 of the right atrial wall. The core biopsy of the mass confirmed that it was metastatic poorly differentiated HCC of thyroidal origin. The patient eventually died of respiratory failure due to a massive pulmonary embolism. For cancer patients with unexplained dyspnea, cardiac metastases should be considered regardless of anticoagulation prophylaxis, especially when there is no deep vein thrombosis in the lower limbs. Early recognition of intracavitary cardiac metastases may help in providing prompt treatment and improving the prognosis.