Goiter is an enlargement of the thyroid gland. It may be diffuse, involving all thyroid tissue, or it may be caused by one or more lumps (nodules)—called nodular goiter. Diffuse goiter reflects an underlying thyroid problem, most commonly iodine deficiency in iodine-deficient areas of the world, where nearly 1 billion people may be subject to the disorder. In the United States, iodine deficiency is rare (because of widespread use of iodized salt) and goiter is most commonly caused by Graves' disease or Hashimoto's disease. Nodular goiter affects 3 to 5 percent of adults, mainly women. Nodules may reflect thyroid cancer (in 5% of cases), but the remainder are benign processes due to multiple causes. Cancers can be distinguished from benign disease by microscopic evaluation of thyroid tissue obtained by fine-needle aspiration biopsy.

MARTIN I. SURKS

(SEE ALSO: Hyperthyroidism; Hypothyroidism; Iodine; Thyroid Disorders; Thyroid Function Tests)