Daily urine iodine excretion while consuming a low-iodine diet in preparation for radioactive iodine therapy in a high iodine intake area

Objective Recommended durations of low-iodine diet (LID) in preparation for radioactive iodine therapy (RAIT) vary among major guidelines and are important for patients in areas where iodine intake is high. The aim of this study was to investigate daily changes in urine iodine excretion after starting a LID. Design The daily iodine/creatinine (I/Cr) ratios and simple iodine concentration (simple I) of morning spot urine from 19 patients with differentiated thyroid carcinoma were measured for 2 weeks from the start of LID for RAIT preparation. We set the cut-off of I/Cr and simple I for poor LID preparation at >66·2 μg/gCr and >150 μg/l, respectively. The day when daily I/Cr or simple I became equal to or below the cut-off both by 95% CI and 90th percentile was defined as the end-point for the appropriate duration of LID for RAIT. Results On day 6 of LID, the I/Cr ratio decreased below the cut-off (≤66.2 μg/gCr) both by 95% CI (0-60·8) and by 90th percentile (51·9). Simple I reached the cut-off (≤150 μg/l) on day 3 by both parameters (95%CI: 2·3-90·5; 90th percentile: 126·5). The morning spot-urine I/Cr and simple I on day 7 and day 14 were significantly lower than on day 0 (P < 0·05). Conclusions One week of a strict LID is enough to decrease the level of urine iodine excretion in preparation for RAIT even in high iodine intake areas. These results provide essential data for future outcome studies regarding LID preparation for RAIT.