Thyroid scanning is used to determine how active the thyroid is in manufacturing thyroid hormone. This can determine whether inflammation of the thyroid gland (thyroiditis) is present. It can also detect the presence and degree of overactivity of the gland (hyperthyroidism) or, conversely, it can determine the presence and degree of underactivity of the gland (hypothyroidism).
Thyroid scanning is helpful in evaluating thyroid nodules, particularly after a fine needle aspiration biopsy of a nodule has failed to provide a definitive diagnosis. A scan will reveal whether a thyroid nodule is functioning or nonfunctioning. A functioning nodule actively takes up iodine to produce thyroid hormone and produces a localized "hot" area on the image of the thyroid gland. A nonfunctioning nodule does not take up iodine and so produces a "cold" area on the image of the thyroid gland. Functioning or "hot" nodules only rarely are from cancer. Nearly all thyroid cancers are nonfunctioning or "cold" nodules. Moreover, even among "cold" nodules, cancer is infrequent (less than 5 percent of cases).