Use of 1-123 in Early Radioiodide Uptake and Its Suppression in Children and Adolescents with Hyperthyroldism

Thyrotoxicosis is characterized by escape of the thyroid gland from the normal regulation of the pituitary-thyroid axis. The thyroidal hyperfunction can be controlled by antithyroid medication, radia tion therapy, or surgery (1,2). When antithyroid medication is well tolerated by the patient, medical management with antithyroid drugs has been con sidered the treatment of choice, especially in chil dren, and medication is terminated when remission is achieved (/). Remission is characterized by re turn of suppressibility of thyroid radioiodine uptake by exogenous triiodothyronine (T3) (3-5). Medical therapy with thionamides depresses thyroid-hormone synthesis by inhibiting the iodination of tyrosyl resi dues and the coupling of the iodinated tyrosyl moi eties after the iodide is trapped by the thyroid gland (6). Since thionamides do not interfere with the trapping of iodide (7), the early thyroidal radioiodine uptake, which depends mostly on the trapping of iodide by the thyroid, is useful in evaluating patients while they are on antithyroid drug treat ment (8). Valuable as this information may be in determin ing whether remission of thyrotoxicosis has occurred, early radioiodine uptake measurement has not gained general acceptance. This is largely because of in herent difficulties in the method and because of re luctance to expose patients, especially children and adolescents, to repeated administration of radioac tive agents (9,70). Use of 1-123 (TV2 = 13 hr) permits considerable reduction in radiation dosage. Introduction of coincidence counting of this nuclide permits the determination of absolute activity with out regard for variation in counting efficiency intro duced by geometric factors, and eliminates the need to use comparisons with an external phantom stand-