RAI is administered by mouth and images are taken to determine where in the thyroid or other parts of the body the RAI is being taken up. This testing provides information not only about the size and shape of the thyroid gland, but also about its function. In addition to I-131, technetium-99m pertechnetate or iodine-123 (other similar radioactive substances) may also be used. A minimal amount of radioactive substance is used, in order to avoid damaging the thyroid gland. Pictures of the thyroid gland are then obtained at varying time periods (hours to days) after the ingestion of these substances (either in pill or liquid form). Thyroid scanning is typically used to evaluate patients for hyperthyroidism or thyroid cancer recurrence.
Hyperthyroidism is a disease in which all or parts of the thyroid are overactive and making too much thyroid hormone. See Hyperthyroidism ». RAI scanning can help determine if the entire gland is hyperactive (usually seen in Graves' disease) or if only specific nodules are hyperactive (i.e. toxic adenoma). If the RAI is taken up by the thyroid more than normal, then the thyroid is considered to be "hot" or overactive. If the RAI is not well absorbed, then the thyroid is called "cold" or underactive. In the scan pictured above, the red areas are "hot," showing that this is a hyperactive thyroid gland.
Thyroid Cancer Recurrence
Patients who have had a total thyroidectomy for thyroid cancer may be sent for RAI scanning with potential ablation. The goal of the scan is to see if there is any remaining normal thyroid tissue (i.e. remnant) or metastatic disease. Even with a total thyroidectomy by an expert surgeon, there is usually a small remnant of thyroid tissue left behind. If patients meet certain criteria, they may be given a larger dose to destroy (i.e. ablate) the remaining thyroid tissue. See Thyroid Cancer ». During their follow-up, if the thyroglobulin level rises, an RAI scan may be ordered looking for recurrent disease. If there is recurrent or metastatic disease, a larger dose of RAI may be given to ablate them.
Of note, radioactive iodine (RAI) scans used to be performed routinely for patients with thyroid nodules because "hot" (i.e. hyperactive nodules) were rarely cancer (less than 1%) while "cold" (i.e. underactive nodules) carried a 10% risk of cancer. However, FNAB is far more accurate for determining the risk of cancer and RAI scans are mostly only used in cases of hyperthyroidism.