Enlargement of the thyroid gland during adolescence should be considered a pathologic rather than physiologic process. With diffuse enlargement in an asymptomatic patient, thyroid function tests and thyroid antibodies usually are the only tests needed to define the diagnosis of euthyroid autoimmune thyroiditis. Patients with lobular or nodular thyroid enlargement may require additional tests if the diagnosis of Hashimoto's thyroiditis cannot be established by the presence of thyroid antibodies in serum. The tests to define the anatomic and functional status of nodular thyroid disease include ultrasonography and radionuclide scintigraphy of the thyroid, but rarely fine needle biopsy. The only indications for surgical therapy of the thyroid are hyperfunctioning thyroid adenomas, the suspicion of thyroid carcinoma and Graves' disease in patients who are not responsive to antithyroid drug therapy and who are poor candidates for radioiodine ablative therapy. The prognosis of thyroid disease during c adolescence is usually excellent.