We report three patients with Down syndrome that developed a hyperthyroidism. A 25 years old female and a 18 years old male had Basedow Graves disease and were treated with radioiodine. A 19 years old male had a Hashitoxicosis and is presently being treated with propylthiouracyl. Clinical and subclinical thyroid dysfunction is frequent in patients with Down syndrome and the risk increases with age. Therefore a surveillance with yearly TSH measurements should be done in these patients, since signs and symptoms of thyroid disease are barely detected in them. Hypothyroidism is the most frequent dysfunction but hyperthyroidism is also associated to Down syndrome.