Bradycardic Tamponade: The Myxedema Pitfall.

Severe, untreated hypothyroidism can produce large, slowly accumulating pericardial effusions; progression to cardiac tamponade is uncommon and may present without tachycardia because of blunted sympathetic tone. We report a middle-aged woman with long-standing hypothyroidism who presented with dyspnea and hypotension but a low resting heart rate. Transthoracic echocardiography demonstrated a large circumferential effusion with right ventricular diastolic collapse and marked respiratory variation in tricuspid inflow, confirming tamponade physiology. She underwent urgent, echo-guided pericardiocentesis with the removal of a large volume of straw-colored fluid, followed by short-term catheter drainage and the initiation of thyroid hormone replacement. Cultures and cytology were negative, and the clinical course, with rapid hemodynamic recovery and resolution of the effusion, supported hypothyroidism as the most likely etiology after excluding infectious, malignant, and autoimmune causes. This case underscores a practical pitfall: bradycardia does not exclude tamponade in severe hypothyroidism. Diagnosis should be anchored in echocardiographic findings, and timely pericardial drainage coupled with endocrine management can achieve rapid stabilization and help prevent recurrence.