Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 1 4 2017 06 24 106 109 EN Zeinab Naderpour Department of Internal Medicine, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran Elnaz Vahidi Department of Emergency Medicine, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran Mahbube Ebrahimpour Department of Internal Medicine, Shariati Hospital, Tehran University of Medical Sciences, Tehran, Iran 2016 03 26 2017 04 19 Mild pericardial effusion is common in severe form of hypothyroidism but massive pericardial effusion with or without cardiac tamponade is rare.  A 49 year-old male with a short stature, large head and facial puffiness, was brought to the emergency department because of progressive altered mental status and severe dyspnea. Cardiac tamponade and hypothyroidism were diagnosed in work ups. Lack of thyroid tissue in his imagines, confirmed the diagnosis of thyroid agenesis. With pericardiocenthesis, hydration and starting levothyroxine his clinical symptoms were resolved dramatically. Our case was presented with full-blown clinical manifestations and irreversible consequences of congenital hypothyroidism. This shows us the important role of newborn screening tests. https://crcp.tums.ac.ir/index.php/crcp/article/view/55 https://crcp.tums.ac.ir/index.php/crcp/article/download/55/57