Although hyperthyroidism is a common endocrine disorder, recognizing it is not always simple. Frequency and severity of symptoms vary significantly from one patient to another. For example, weight loss despite good appetite is a classic manifestation of thyrotoxicosis, but some patients, particularly those who are young, gain weight. In addition, manifestations are sometimes atypical. For instance, older patients may present with only atrial fibrillation, heart failure, or weight loss and cachexia. In this article, Dr Haddad describes how to distinguish euthyroid hyperthyroxinemia from true thyroid disease and summarizes how to choose among treatment approaches.