Impact of 131I diagnostic activities on the biokinetics of thyroid remnants.
Recent publications described many discrepant findings about thyroid "stunning" after the administration of (131)I diagnostic activities to patients with differentiated thyroid carcinoma. Stunning may play a major role in reducing the therapeutic efficacy of high (131)I 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 (131)I twice within 6 wk and an ablation activity of 3-4 GBq 7-12 d after the second diagnostic administration of (131)I in 6 patients.


RESULTS
The uptake and half-life of the first 74 MBq of (131)I 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.


CONCLUSIONS
This study shows a severe impact of 74 MBq of (131)I on the biokinetics of thyroid remnants during subsequent radioiodine therapy.
Lassmann
Michael
M
Klinik und Poliklinik für Nuklearmedizin der Universität Würzburg, Würzburg, Germany. lassmann@nuklearmedizin.uni-wuerzburg.de
Luster
Markus
M
Hänscheid
Heribert
H
Reiners
Christoph
C
D000328:Adult; D005260:Female; D006207:Half-Life; D006801:Humans; D007037:Hypothyroidism / Q000209:etiology / Q000378:metabolism*; D007457:Iodine Radioisotopes / Q000009:adverse effects / Q000493:pharmacokinetics* / Q000627:therapeutic use; D008297:Male; D008657:Metabolic Clearance Rate; D008875:Middle Aged; D011877:Radionuclide Imaging; D011994:Recombinant Proteins / Q000008:administration & dosage; D012074:Remission Induction; D013961:Thyroid Gland / Q000378:metabolism* / Q000528:radiation effects*; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism / Q000628:therapy*; D013965:Thyroidectomy / Q000009:adverse effects; D013972:Thyrotropin / Q000008:administration & dosage
D007457:Iodine Radioisotopes; D011994:Recombinant Proteins; D013972:Thyrotropin