Case Summary: A 52-year old male was admitted at the Intensive Care Unit of Ospital ng Makati due to difficulty of breathing. The patient is a diagnosed case of papillary thyroid carcinoma, S/P total thyroidectomy and radioactive iodine treatment (1997) maintained on levothyroxine 100 mcg OD with poor compliance. On admission, the patient had neck vein engorgement, bibasal crackles, muffled heart sounds, abdomen had shifting dullness and (+) fluid wave test, and grade 3 bipedal edema. 12-L ECG showed electrical alternans and chest x-ray revealed an enlarged heart with water bottle configuration. Echocardiography showed compressed right ventricle and right atrium, right atrium systolic indentation, and a large echo free space which signified cardiac tamponade. The patient was tachypneic and became hypotensive. Subxiphoid pleuropericardial window with pericardiocentesis was done draining a total of 1.8 liters serosanguinous fluid. Thyroid function tests showed elevated TSH (>40uIU/L), decreased F T4 (0.00ng/dL ) and F T3 (0.72pg/mL ) . levothyroxine 25 mcg OD was started and increased to 50 mcg after three days. Pericardial fluid showed predominance of lymphocytes and had high protein content. The dyspnea was relieved and patient had increased urine output and decreased edema after the pericardiocentesis. Repeat 12-L ECG four hours after pericardiocentesis showed resolution of the electrical alternans. Repeat 2D-Echo with Doppler after several days showed mild pericardial effusion. Repeat thyroid function tests showed normal FT4 and FT3 levels and increased TSH (34.5uIU/L). The patient was discharged improved maintained on levothyroxine 100 mcg OD and had no recurrence of the cardiac tamponade. Conclusion: Cardiac tamponade is a rare complication of hypothyroidism. A high index of suspicion is needed for prompt diagnos i s of cardiac tamponade and hypothyroidism followed by immediate intervention. Pericardiocentesis is necessary and hormonal treatment with levothyroxine is crucial as it produces satisfactory clinical outcome with reduction in recurrences of the cardiac tamponade after drainage is performed.