Autoimmune thyroiditis with hypothyroidism is a rarely reported feature of Noonan syndrome1). Cardiac tamponade which is a medical emergency can be the first symptom of autoimmune thyroiditis associated with Noonan syndrome. Cardiac tamponade is extremely infrequent in hypothyroid patients with Hashimoto thyroiditis. Adolescent patients with Noonan syndrome and Hashimoto thyroiditis presenting with cardiac tamponade have not been reported previously. A 16-year-old male was demonstrated with massive pericardial effusion. He was also diagnosed of hypothyroidism due to Hashimoto thyroiditis on the same day. The patient was treated with L-thyroxine at 0.15 mg daily. However, after admission, he presented symptoms of cardiac tamponade and he underwent closed pericardiostomy which yielded up to 360 ml of serous, yellowish clear pericardial fluid. After closed pericardiostomy, the patient’s chest discomfort improved and his vital signs stabilized. Over a 6-month follow-up period, levels of TSH and free T4 normalized on L-thyroxine medication. The patient is still on medication. On chest X-ray, 6 months after the pericardial drainage, the cardiothoracic ratio was normal and there was no evidence of the cardiomegaly. In patient with cardiomegaly, thorough physical examination is essential and clinicians should be aware of the possibility of concurrent autoimmune conditions in patients with Noonan syndrome. Also, appropriate clinical and laboratory evaluations should be performed, considering that pericardial effusion or cardiac tamponade can be the first symptom of autoimmune disease.