The use of 123I (half-life of 13,3 h, no beta emission) results in a marked diminution of thyroid and total body exposure in comparison to 131I. 99mTc-pertechnetate would give the lowest radiation dose, but is not an ideal thyroid agent because it only reflects the iodine trapping mechanism. Several studies show discrepancies between pertechnetate and iodine scans, i.e. Usher and Arzoumanian (2), Turner and Spencer (3). 123I is cyclotron produced, in Switzerland by the 127I (p, 5n) leads to 123Xe leads to 123I reaction. The short half-life needs a modification of the usual testing with 131I. In the present study, thyroid uptakes after 6 h and 24 h, blood sampling after 24 h for 123PBJ and scanning after 6 h give the best results for a complete 123I tracer study of the thyroid. If only morphology of the thyroid is needed, a 6 h scan is done. The normal ranges for uptake values and the 123PBJ are similar to those with 131I. The scans with 123I are definitely faster and of better quality, either by scanner or by camera. The administered doses are 100 muCi for scanning and 300 muCi 123I orally for a complete tracer study. 100 muCi 123I result in a thyroid radiation dose of about 1,5 rads and a total body dose of about 3 mrads [James, Wagner, Cook (1)].
