Pericardial effusion is commonly seen in patients with hypothyroidism, but a massive pericardial effusion with obvious diastolic right ventricular compression is uncommon. We herein report a case of 73-year-old woman seen in the ED with generalized weakness and hypotension. Echocardiography revealed a massive pericardial effusion with diastolic right ventricular compression, and thyroid function testing revealed marked hypothyroidism. The pericardial effusion resolved after the administration of thyroid replacement therapy. This case reveals the importance of including hypothyroidism in the differential diagnosis of pericardia effusion.