Two Weeks of a Low-Iodine Diet Are Equivalent to 3 Weeks for Lowering Urinary Iodine and Increasing Thyroid Radioactive Iodine Uptake

Abstract

The importance of restricting iodine intake to increase thyroid uptake of ¹³¹I is well-known, but its minimum duration is not clear. The present study aimed at determining whether 3 weeks is superior to 2 weeks for a low-iodine diet (LID) as a means of increasing the thyroid uptake of ¹³¹I and reducing urinary iodine. A second goal was to evaluate the effectiveness of a questionnaire about ingestion and exposure to iodine to predict individuals in whom the LID would be less effective. Forty-six outpatients who came to the Nuclear Medicine Unit of Santa Casa de Porto Alegre, Brazil, for thyroid evaluation, were randomized to receive an LID for 2 or 3 weeks. Questionnaires regarding their exposure to iodine, the thyroid uptake of ¹³¹I, and the iodine and iodine/creatinine ratio in urine samples were administered or obtained at the beginning and end of the diet. The questionnaire indicated exposure to exogenous iodine in five patients during their LID, all in the 3-week group. Their thyroid ¹³¹I uptake was significantly lower than patients who did not give a history of iodine exposure (p = 0.016). The comparative analysis between the 2-week and 3-week diet groups was then performed excluding the five contaminated patients. The ¹³¹I uptake increased by 43% in the 2-week group and 26.7% in the 3-week group (p = 0.105). Both diets for 2 and 3 weeks caused significant decrease in urinary iodine (p < 0.001), without a difference between the groups (63.2% in the 2-week group and 60.9% in the 3-week group, p = 0.955). There was no difference in the percentage of patients with urinary iodine ≤100 μg/L (p = 0.25) and urinary iodine ≤50 μg/L (p = 0.86) between the groups. A correlation between urinary iodine and iodine/creatinine ratio (r = 0.516; p < 0.001) was observed. Two weeks of an LID are probably sufficient to augment thyroid uptake of ¹³¹I, with little or no benefit from longer periods of an LID. Questionnaires regarding exposure to iodine similar to those employed here should identify individuals in whom the LID has not been as effective in increasing the thyroid uptake of ¹³¹I.