Low iodine diet for one week is sufficient for adequate preparation of high dose radioactive iodine ablation therapy of differentiated thyroid cancer patients in iodine-rich areas

Background: Most current guidelines suggest one or two weeks of low iodine diet (LID) before radioactive iodine ablation therapy (RAIT) to increase its efficacy in differentiated thyroid cancer (DTC) patients after total thyroidectomy. LID duration is particularly important for patients living in iodine excess areas. However, there is no standardized LID protocol and there are limited reports regarding the relationship between LID and ablation outcome. Therefore, we aimed to evaluate the optimal LID duration and define clinical features that affect ablation outcome. Methods: A total of 202 papillary thyroid cancer patients with total thyroidectomy preparing for RAIT were enrolled. All patients had undergone two weeks of LID before ¹³¹I administration. Morning spot urine specimens were obtained twice (one week or two weeks after LID, respectively) from each patient. Urine iodine excretion (UIE) values were used to evaluate LID efficacy. Successful ablation was defined using two definitions: (i) no visible uptake on a follow-up diagnostic ¹³¹I scans, and (ii) no visible uptake on a follow-up diagnostic ¹³¹I scans and stimulated serum thyroglobulin (Tg) levels <1ng/mL. Results: The UIE median values after LID for one and two weeks were lower than 50μg/L, and the median UIE values were not significantly different according to the LID duration. Based on the first criterion for successful ablation, 175 of the 195 patients were successfully ablated. There were no significant differences in mean and median UIE levels between the ablated and non-ablated groups after LID for two weeks. The rate of ablation did not differ between the mild and moderate iodine deficient groups. Based on the second criterion for successful ablation, 149 of 188 patients were successfully ablated. The ablation success rate did not differ between UIE levels. When we analyzed clinical factors that affect ablation outcome, serum Tg level at the time of ablation was the only significant variable in multivariate logistic analysis. Conclusion: Strict LID for one week was sufficient to achieve target UIE values for RAIT preparation, even in iodine-rich areas.