The thyroid scan is a nuclear medicine study performed to assess thyroid size, shape, position, and function. It is useful for evaluating thyroid nodules, multinodular goiter, and thyroiditis; assisting in the differential diagnosis of masses in the neck, base of the tongue, and mediastinum; and ruling out possible ectopic thyroid tissue in these areas. Thyroid scanning is performed after oral administration of radioactive iodine-123 (I-123) or I-131 or IV injection of technetium-99m (Tc-99m). Increased or decreased uptake by the thyroid gland and surrounding area and tissue is noted: Areas of increased radionuclide uptake (“hot spots”) are caused by hyperfunctioning thyroid nodules, which are usually nonmalignant; areas of decreased uptake (“cold spots”) are caused by hypofunctioning nodules, which are more likely to be malignant. Ultrasound imaging may be used to determine if the cold spot is a solid, semicystic lesion or a pure cyst (cysts are rarely cancerous). To determine whether the cold spot depicts a malignant neoplasm, however, a biopsy must be performed.