Abstract. Thyroid stunning is a phenomenon of impaired thyroid tissue function after administration of a therapeutic dose of 131I subsequent to a diagnostic dose of 111- MBq (3 mCi) or greater. The impact of stunning on the clinical outcomes is still not well documented. Aim: The purpose of this study was to investigate the clinical effects of stunning. Methods: Four to six weeks after total or near-total thyroidectomy forty patients with nonmetastatic papillary or follicular carcinoma of the thyroid were evaluated. TSH level should be above 30 mU/L. They were divided prospectively into 2 groups. Group 1: 20 patients underwent 131I ablative therapy without a preliminary 131I diagnostic whole body scan. Group 2: 20 patients underwent 131I ablative therapy after preliminary 131I diagnostic whole body scan. Comparisons of the postablation outcomes were evaluated by chi (2) analysis. Successful ablation required a negative follow-up thyroid scan 6 months after ablation and also thyroglobulin level < 1.5 μg/L. Results: Only 30% of patients in the scanned group had the thyroid gland ablated after a single 131I dose, compared with 85% in the non scanned group (P<0.005). 30% of patients in scanned group versus 15% in non-scanned group required second therapeutic dose. 40% of patients in scanned group required third therapeutic dose. In contrast no patients received third dose in the non-scanned group. Patients in the scanned group required higher total 131I therapeutic activity (199mCi or 7.36GBq) to ablate the thyroid gland compared to those in the non- scanned group (132 mCi or 4.8 GBq). In analysis of the relation between TSH level and number of patients required single 131I dose, to achieved thyroid remnant ablation, there are 5 patients in the range from 30 to 40 mU/l, 8 patients in the range of 41 to 50 mU/l and 10 patients in the range of 51 to 61 mU/l. The
difference in outcome between the scanned and the non- scanned groups demonstrates that the efficacy of 131I ablation is reduced subsequent to the use of 185 MBq (5 mCi) of 131I for diagnostic imaging. Conclusions: this study confirms that thyroid stunning after diagnostic 131I whole body scan affect the clinical outcomes of 131I ablation. We also concluded that the higher the TSH level the greater the number of ablated thyroid remnant after single 131I therapeutic dose.