Iodine supplementation of mildly iodine-deficient adults lowers thyroglobulin: A randomized controlled trial

Abstract

Context: An inverse relationship between thyroglobulin (Tg) and urinary iodine concentration (UIC) has been found in children, potentially making Tg a viable blood marker of iodine status. The application of Tg in adults is unknown. Objective: The objective of the study was to determine the efficacy of Tg to assess iodine status in adults. Design: This was a randomized, double-blind, placebo-controlled, clinical trial. Setting: The study was conducted in Dunedin, New Zealand. Participants: Mildly iodine deficient adults (n = 112) aged 18-40 years participated in the study. Intervention: Participants were supplemented with 150 μg of iodine as potassium iodate or placebo daily for 24 weeks. At baseline and 24 weeks, participants provided five casual urine samples for UIC determination; serum TSH and free T4 (FT4) was also measured. Tg was determined at baseline and 8, 16, and 24 weeks. Main Outcome Measure: A change in Tg concentration between the iodine-supplemented and placebo groups at 24 weeks. Results: At baseline, the overall median UIC was 65 μg/L, confirming that participants were mildly iodine deficient (ie, median UIC between 50 and 99 μg/L). The overall median Tg was 16.6 μg/L; TSH and FT4 were within normal reference ranges. At 24 weeks, the median UIC of the placebo and supplemented groups was significantly different, 79 and 168 μg/L, respectively (P < .001). Tg in the iodine-supplemented group decreased by 12%, 20%, and 27% compared with the placebo group at 8 (P = .045), 16 (P < .001), and 24 weeks (P < .001); there were no significant changes in TSH and FT4. Conclusion: Improved iodine status as assessed by UIC was associated with a concomitant decrease in Tg concentration, demonstrating that Tg is a useful biomarker of iodine status in a group of adults.