A 65-year-old woman with a history of multinodular goiter complains of increasing nervousness, insomnia, and heart palpitations. She has lost 9 kg (20 lb) over the past 6 months. Physical examination reveals a diffusely enlarged thyroid. There is no evidence of exophthalmos. Laboratory studies show elevated serum levels of T3 and T4. Serologic tests for antithyroid antibodies are negative. Which of the following is an important complication of this patient's endocrinopathy?

Hyperthyroidism refers to the clinical consequences of an excessive amount of circulating thyroid hormone. The principal metabolic products of the thyroid gland are triiodothyronine (T3) and tetraiodothyronine (thyroxine; T4). T4 is principally a prohormone; the major effector of thyroid function is T3. These molecules are formed by the iodination of tyrosine residues of thyroglobulin within the follicular cells. Iodinated thyroglobulin is then secreted into the lumen of the follicle. Many patients with nontoxic goiter, usually over the age of 50 years, eventually develop a toxic form of the disease. Since patients with toxic goiter tend to be older, cardiac complications, including atrial fibrillation and congestive heart failure, dominate the clinical presentation.Diagnosis: Hyperthyroidism, toxic goiter