RT Journal Article SR Electronic T1 Impact of 131I Diagnostic Activities on the Biokinetics of Thyroid Remnants JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 619 OP 625 VO 45 IS 4 A1 Michael Lassmann A1 Markus Luster A1 Heribert Hänscheid A1 Christoph Reiners YR 2004 UL http://jnm.snmjournals.org/content/45/4/619.abstract AB Recent publications described many discrepant findings about thyroid “stunning” after the administration of 131I diagnostic activities to patients with differentiated thyroid carcinoma. Stunning may play a major role in reducing the therapeutic efficacy of high 131I activities given for ablation therapy. Methods: Participation in a multicenter study to investigate differences in iodine biokinetics in the hypothyroid state and after the application of recombinant human thyroid-stimulating hormone enabled us to study quantitative changes in thyroid iodine biokinetics after the administration of 74 MBq of 131I twice within 6 wk and an ablation activity of 3–4 GBq 7–12 d after the second diagnostic administration of 131I in 6 patients. Results: The uptake and half-life of the first 74 MBq of 131I were significantly reduced to a mean of 44% and a mean of 51%, respectively, after the second diagnostic administration and further reduced to a mean of 40% and a mean of 30%, respectively, during ablation therapy. The residence times were reduced to 25% in the second dosimetric assessment and to 10% during therapy compared with the value in the first assessment. For one patient, an estimated absorbed dose as high as 38 Gy was found in the first diagnostic study. The mean dose for all patients after the first assessment was 15 Gy; after each further assessment, the dose was reduced according to the decrease in residence time. Conclusion: This study shows a severe impact of 74 MBq of 131I on the biokinetics of thyroid remnants during subsequent radioiodine therapy.