Is Two Weeks Low Iodine Diet Superior to One Week Low Iodine Diet in Decreasing Urinary Iodine Concentration in Patients Undergoing Rhtsh-Aided Radioiodine Ablation for Differentiated Thyroid Cancer?

49 Objectives: Low iodine diet (LID) is an important preparation for radioactive iodine remnant tissue ablation (RIA) for differentiated thyroid cancer. However, strict adherence to LID for a longer period is not easy for patients living in an iodine sufficient area. The aim of this study is to examine whether a LID for two weeks is superior to a LID for one week in decreasing urinary iodine concentration (UIC), which is supposed to be a biomarker for iodine status. Methods: We enrolled 38 patients (M/F 7/26. Age 61.2±14.2yrs.) with post-surgical papillary thyroid cancer with stage II or stage III by TNM classification 8th edition, who were to undergo rhTSH aided RIA with 1.11 GBq of I-131. After Interview and counselling with a certified dietitian, all patients followed LID (< 100 μg/day) for two weeks prior to administration of I-131. UIC normalized by urinary creatinine (μg/gCRE) was measured three times using a spot urine sample: before LID (UIC1), one week after LID (LID2), and two weeks after LID (UIC3). Based upon the definition by WHO, UIC < 100(μg/gCRE) were considered as iodine depletion status. I-131 uptake in the residual tissue (RIU) was determined 72 hrs after RAI using a gamma camera. Response assessment was done 6-12 mos. after RAI by combination of diagnostic dose I-131 SPECT/CT, neck US, and stimulated Tg test. Nine patients with positive TgAb values additionally underwent FDG-PET/CT. Fulfillment of negative I-131 uptake, no suspicious mass on anatomical imaging, and stimulated Tg value < 1.0 (ng/ml) or no abnormal FDG uptake in patients with positive TgAb was considered as complete response (CR). Results: One week after start of LID, the average value (μg/gCRE) of UIC2 dropped to 17% of that of UIC1(78.8 vs. 463.7, p=0.0091). However, two weeks after LID, the average of UIC3 was not further decreased and were rather higher than that of UIC2. (130.2 vs. 78.8, p=0.0416) Iodine depletion status were archived in 84% of the patients (32/38) after one-week LID, while It was observed in only 55%of patients (21/38) after two weeks LID Patients were further divided into 2 subgroups by UIC3: A) 16 patients in whom UIC3 was lower than UIC2, and B) 22 patients in whom UIC3 was equal to or higher than UIC2. There was no statistical difference in the 73hrs RIU between group A vs. group B (4.8 vs. 4,1, p=0.1957), while CR after RAI was more frequently achieved in group A than in group B (88% vs 50%. p=0.01276) Conclusions: A two-weeks LID failed to show advantage over a one-week LID in reducing UIC or inducing iodine depletion in patients undergoing rhTSH -aided RAI. Our finding strongly suggests that strict adherence to LID for a longer duration does not guarantee lower iodine concentration in the body prior to RAI. However, patients had lower UIC3 than UIC 2 may be associated better outcome from RAI compared with those who had higher UIC3 than UIC2.