Low-Iodine Diet of 4 Days Is Sufficient Preparation for I-131 Therapy in Differentiated Thyroid Cancer Patients

Context: No consensus exists about the optimal duration of the low-iodine diet (LID) in the preparation of I-131 therapy in differentiated thyroid cancer (DTC) patients.

Objective: This work aimed to investigate if a LID of 4 days is enough to achieve adequate iodine depletion in preparation for I-131 therapy. In addition, the nutritional status of the LID was evaluated.

Methods: In this prospective study, 65 DTC patients treated at 2 university medical centers were included between 2018 and 2021. The patients collected 24-hour urine on days 4 and 7 of the LID and kept a food diary before and during the LID. The primary outcome was the difference between the 24-hour urinary iodine excretion (UIE) on both days.

Results: The median 24-hour UIE on days 4 and 7 of the LID were not significantly different (36.1 mcg [interquartile range, 25.4-51.2 mcg] and 36.5 mcg [interquartile range, 23.9-47.7 mcg], respectively, P= .43). On day 4 of the LID, 72.1% of the DTC patients were adequately prepared (24-hour UIE < 50 mcg), and 82.0% of the DTC patients on day 7 (P= .18). Compared to the self-reported regular diet, DTC patients showed a significantly (P< .01) lower percentage of nutrient intake (calories, protein, calcium, iodine, and water) during the LID.

Conclusion: The 24-hour UIE on day 4 of the LID did not differ from day 7, and therefore shortening the LID from 7 to 4 days seems justified to prepare DTC patients for I-131 therapy in areas with sufficient iodine intake and may be beneficial to maintain a sufficient nutritional intake during DTC treatment.