Pericardial Effusion in the Emergency Department.

We report the first child presenting to the emergency department with undiagnosed myxedema in whom incidental detection of pericardial effusion led to diagnosis. Moreover, this patient presented with the highest serum thyrotropin concentration reported to date, a phenomenon that caused the hook effect during laboratory analysis. We discuss key elements of the recognition and management of hypothyroidism in the pediatric population and emphasize the importance of annual screening for hypothyroidism in all patients with Down syndrome.