"Thyroid stunning" from diagnostic iodine-131 imaging prior to ablative therapy with (131)I for well-differentiated thyroid carcinoma has been well reported, but documentation of the effect on clinical outcome is sparse. The purpose of this retrospective study was to investigate the clinical effects of stunning. The outcome of (131)I ablative therapy in a group of patients ( n=36) who had diagnostic scans using 185 MBq (5 mCi) of (131)I was compared with that in a group ( n=36) who had diagnostic scans using 740 MBq (20 mCi) of (123)I. Patients were imaged at least 4 weeks after near-total thyroidectomy, prior to their first (131)I ablative therapy. Follow-up imaging was performed every 3-6 months, and further (131)I treatment administered when indicated. A group of patients ( n=36) who proceeded directly to their first therapy dose without a diagnostic scan and were followed up with (123)I was compared with the group who did have a (123)I diagnostic scan prior to the first ablative therapy. The efficacy of therapy was evaluated using ablation of the thyroid, evidenced by absence of uptake in the thyroid bed on the diagnostic scan, as the endpoint. Only 47% of patients in the (131)I diagnostic group had the thyroid gland ablated after a single administration of (131)I therapy, compared with 86% in the (123)I diagnostic group ( P<0.005). Patients who had (131)I diagnostic scans required higher total (131)I therapeutic activity (6.7 GBq or 180 mCi) to ablate the thyroid gland than those in the (123)I diagnostic group (4.4 GBq or 119 mCi). There was no difference in outcome between the group who did and the group who did not have a diagnostic study with (123)I prior to their first ablative therapy. The difference in outcome between the (131)I and the (123)I diagnostic groups demonstrates that the efficacy of (131)I therapy is reduced subsequent to the use of 185 MBq of (131)I for diagnostic imaging. This indicates that the phenomenon of stunning is clinically significant and affects the outcome of therapy.

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