Hyperthyroidism and thyrotoxicosis are terms that are used interchangeably by most clinicians, yet they can mean different things. Hyperthyroidism is defined as a process causing excess production and secretion of thyroid hormones by the thyroid gland, while thyrotoxicosis often refers to the clinical signs of a hypermetabolic state. An increase in sympathetic nervous system symptoms usually characterizes this hypermetabolic state. It should be noted that clinical manifestations of thyrotoxicosis can differ based on age, and that the degree of biochemical abnormality does not always correlate with symptom presentation. The major causes of hyperthyroidism are summarized in Table 10.1, with the vast majority of cases being secondary to Graves’ disease, toxic multinodular goiter (TMG), and toxic adenoma.