Although relatively rare, hypothyroidism remains a significant cause of moderate to severe pericardial effusion. Pericardial effusion secondary to hypothyroidism does not usually cause symptoms since it tends to regress slowly and ultimately disappear several months after the patient has reverted to the euthyroid state. Thus, hypothyroidism must be ruled out in patients with an unexplained pericardial effusion, both to improve prognosis and to avoid unnecessary pericardiocentesis. Even when they have a massive pericardial effusion, patients should receive the standard treatment for hypothyroidism. We herein describe a 79-year-old woman with a massive pericardial effusion associated with hypothyroidism who showed a good response to standard levothyroxine replacement therapy after 5months.