The Urinary Iodine Level Based on Different Duration of Low-Iodine Diet for Radioactive Iodine Therapy

The optimal period of a low-iodine diet (LID) during preparation for radioactive iodine therapy (RAI) is not well established. At the University of Washington, we conducted a retrospective analysis of patients with a diagnosis of differentiated thyroid cancers who underwent total thyroidectomy and RAI during the period 2022-2024 and compared urine iodine concentration in patients who had either 1 or 2 week of LID prior to RAI.

All analyses were performed using SAS software 9.4 (SAS Institute). There were 72 patients who met the criteria. The median age of this cohort was 46 years (range: 19-82) and 63.9% were female. Of these, 53 patients underwent a 1-week and 19 patients underwent a 2-week LID before RAI. The median value of urine iodine after a 2-week restriction was lower at 0.228 μmol/L (29 μg/L) (range: <0.079-1.481 μmol/L; <10-188 μg/L) compared to 1 week at 0.362 μmol/L (46 μg/L) (range: <0.079-30.43 μmol/L; <10-3863 μg/L), but not statistically significant (P = .24).

Although the urine iodine level was lower in 2-week restriction, there was no statistically significant difference between 1 versus 2-week iodine restrictions following RAI. This suggests that a 1-week LID may be adequate for RAI treatment. This would decrease time to treatment for patients and decrease the burden of LID.

There is a need for future large prospective randomized control trials to identify the optimal duration of LID and determine the goal urinary iodine level for successful RAI ablation.