Autonomously functioning thyroid nodules (AFTNs) are presumably independent of TSH for growth and function and appear "hot" on scintiscan because they selectively concentrate radionuclide to a greater extent than the remaining thyroid gland, which is controlled by the normal TH-TSH feedback mechanism. Such autonomously functioning tissue may occur in "patchy" areas, as a solitary nodule, or as multiple nodules (classic Plummer's disease), with the mass of hyperfunctioning tissue and the related secretion of thyroid hormones determining whether the patient is euthyroid or hyperthyroid. Important diagnostic tests include a 99mTc thyroid scan, T4 RIA, T3 uptake, FTI, TSH RIA, and occasionally T3 RIA ("T3 thyrotoxicosis"). Solitary autonomous nodules in adult patients characteristically progress slowly over many years, with toxicity rarely developing in nodules less than 2.5 cm in diameter and occurring primarily in nodules 3 cm or larger and in older patients. The decision to treat a solitary nodule depends upon the size and degree of function of the nodule and the patient's age. Surgery and radioactive iodine are effective therapies. Hyperfunctioning thyroid nodules in children and adolescents (under age 18) have a more rapidly progressive course than those in adults and should be treated by thyroid lobectomy at the time of diagnosis. Subtotal thyroidectomy is the preferred treatment for most patients with toxic multinodular goiter, because it achieves prompt control of the hyperthyroidism and removes the goiter. Radioiodine therapy and long-term antithyroid drug therapy are alternative forms of treatment for patients who are poor surgical risks or who develop recurrent hyperthyroidism following thyroid surgery.