Effect of a Diagnostic Dose of 185 MBq 131I on Postsurgical Thyroid Remnants

Stunning of thyroid remnants after diagnostic scanning with 131I may limit 131I therapy. The follow-up scans and serum thyroglobu-lin levels of such patients have been studied. Methods: Three-hundred seventy-eight patients who underwent thyroidectomy for differentiated thyroid carcinoma were studied. Diagnostic scans were obtained with 185 MBq 131I; 7.2 wk afterward, the patients received 489 ablative treatments with 4 GBq 131I. A total of 1575 scans were obtained (pre- and posttreatment scans and 615 follow-up scans). The evolution of serum thyroglobulin levels was determined. Results: In all patients, the thyroid-stimulating hormone level was more than 30 µIU/mL. Posttherapy scans showed less uptake than did diagnostic scans or even showed negative findings in 99 patients (21%). In these patients, the mean time between diagnostic scanning and therapy was 7.9 wk. In 61 of these 99 patients (61.6%), follow-up scans have shown negative findings, serum thyroglobulin was less than 3 ng/mL, and antithyroglobulin antibodies were not present. Twenty-three patients (23.2%) have not undergone follow-up scanning yet. In 8 patients (8.1%), follow-up scans showed negative findings but serum thyroglobulin was more than 3 ng/mL. In 7 patients (7.1%), follow-up scans showed less uptake than posttherapy scans, but the findings were not yet negative and thyroglobulin was less than 3 ng/mL. Conclusion: Our data suggest that a stunning effect does not exist for doses of 185 MBq 131I for diagnostic scans. However, a therapeutic effect may exist. Key Words: stunning effect; diagnostic dose; 131I treatment; differentiated thyroid carcinom