Abstract
Down syndrome (DS) is the most common chromosomal disorder in live-born infants, often associated with intellectual disability and various medical conditions, including thyroid dysfunction. Hashimoto's thyroiditis (HT), an autoimmune subtype, is a leading cause of acquired hypothyroidism in DS children. Severe hypothyroidism can precipitate myxedema, a critical condition linked to complications like pericardial effusion and cardiac tamponade. This case study presents a nine-year-old male with DS who was admitted for acute respiratory distress exhibiting classic signs of myxedema. Initial investigations revealed severe hypothyroidism and significant pericardial effusion. Surgical pericardiotomy drained 800 mL of fluid, confirming myxedema secondary to HT. Levothyroxine therapy led to progressive improvement, resolving myxedematous infiltrate and associated symptoms within a month. Follow-up at 12 months demonstrated sustained improvement with normalized thyroid function and no clinical disease activity. This case highlights an atypical presentation of HT in a DS child with cardiac pre-tamponade.