Two-Week Low Iodine Diet Is Necessary for Adequate Outpatient Preparation for Radioiodine rhTSH Scanning in Patients Taking Levothyroxine

Abstract

We evaluated a self-managed, outpatient, low iodine diet (LID) (<50 μg/day) designed to decrease total body iodine (TBI) in preparation for radioiodine (¹³¹I) scans. However, levothyroxine(LT₄) ingestion is a significant source of dietary iodine in recombinant human TSH (rhTSH)-stimulated studies. Measuring urine iodine to creatinine ratios (I/Cr), a reflection of TBI, we evaluated the LID for 7-day and 14-day periods to determine the efficacy of our LID to deplete TBI, with and without LT₄. Patients following the LID for 14 days (n = 28) without LT₄ attained the goal of an iodine deficient state (I/Cr <50 μg/g) in 78% of cases, establishing the diet's efficacy in significantly reducing TBI. In patients taking LT₄, 7 days of the LID was insufficient to attain this goal of true iodine deficiency. However, a 14 day LID while taking LT₄ resulted in 21% of patients being iodine deficient. For diagnostic purposes, a 7 day LID period (n= 21) suboptimally but adequately (I/Cr <100 μg/g) prepared 41% of the patients, whereas 14 days on the diet (n = 24) adequately prepared 71% of the patients taking LT₄. Our simple, self-managed, outpatient, LID effectively makes patients iodine deficient. Though less efficacious when taking LT₄, this LID adequately reduces TBI for rhTSH-stimulated ¹³¹I uptake scans when followed for 14 days.