Thyroid scan: An image taken of a patient's thyroid gland after the patient swallows radioactive iodine or technetium. The image shows the thyroid gland in action as it accumulates radioactive material. Thyroid scanning is used to determine how active thyroid tissue is in manufacturing thyroid hormone. This can help a physician determine whether inflammation of the thyroid gland (thyroiditis) is present. It can also show the presence and degree of overactivity of the gland (hyperthyroidism). Thyroid scanning is especially helpful in evaluating thyroid nodules, particularly after a fine-needle aspiration biopsy has failed to provide a diagnosis. A scan can reveal whether a thyroid nodule is functioning. A functioning nodule actively takes up iodine to produce thyroid hormone, and so it produces a localized 'hot' area on the image. A nonfunctioning nodule does not take up iodine, and it produces a localized 'cold' area. Most nodules, particularly if they are functioning, are not malignant.