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 microg/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 microg/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 microg/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.