Abstract

Radioactive iodine I-131 is a radionuclide that emits both gamma and beta radiation and, for this reason, has long been used in the diagnosis and treatment of benign and malignant thyroid diseases. I-131 whole-body scan has been the most commonly used imaging method in clinical practice for the follow-up of differentiated thyroid cancer (DTC) owing to the concentration of iodine in differentiated functional thyroid tissue, a high target-to-background activity ratio, and its specificity. However, the fact that serum thyroglobulin and anti-thyroglobulin levels are markers as specific for DTC as RAI uptake (RAIU), and that ultrasonography enables successful screening for cervical lymph node metastases and local recurrent tissue, has considerably limited the use of I-131 scan, particularly in the low-risk patient group. The RAIU test, on the other hand, reflects radiopharmaceutical uptake and kinetic processes within the thyroid gland, thereby enabling quantification of thyroid iodine metabolism; it is important in the differential diagnosis of thyrotoxicosis and, especially in hyperthyroidism, in planning the dose for RAI therapy.