Introduction: Hypothyroidism is a multisystem disorder with diverse clinical manifestations. Pericardial effusion is a rare but potentially life-threatening complication of severe hypothyroidism. Early diagnosis and prompt treatment are crucial to prevent morbidity . Case presentation: A 14-year-old girl, born to non-consanguineous parents and previously well,presented with lethargy , weight gain, exertional dyspnea, and declining school performance for one year. Examination revealed short stature (height 100 mIU/L) with very low free T4 and positive anti-thyroid peroxidase antibodies. Neck ultrasonography was consistent with chronic thyroiditis. She had normocytic normochromic anemia, severe hypercholesterolemia (LDL 570 mg/dL), and elevated creatine phosphokinase (1270 U/L). Chest radiography suggested pericardial effusion, confirmed by echocardiography showing a massive effusion measuring 21 mm. Bone age corresponded to 10 years. She was commenced on oral thyroxine therapy with close biochemical and echocardiographic monitoring. Follow-up showed marked clinical improvement and complete resolution of the pericardial effusion without invasive intervention. Conclusion: Severe untreated autoimmune hypothyroidism may present with significant multisystem complications. Timely recognition and thyroid hormone replacement therapy can result in full recovery and avoid surgical management.