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ORIGINAL RESEARCH COMMUNICATION |
1 From the Human Nutrition Laboratory, Swiss Federal Institute of Technology, Zürich, Switzerland (MBZ and SYH); the Department of Pediatrics, Asahikawa Medical College, Asahikawa, Japan (YI and KF); and the Department for Growth and Development, University Children's Hospital, Zürich, Switzerland (LM).
Background: There are few data on the adverse effects of chronic exposure to high iodine intakes, particularly in children.
Objective: The objective of the study was to ascertain whether high dietary intakes of iodine in children result in high thyroid volume (Tvol), a high risk of goiter, or both.
Design: In an international sample of 6–12-y-old children (n = 3319) from 5 continents with iodine intakes ranging from adequate to excessive, Tvol was measured by ultrasound, and the urinary iodine (UI) concentration was measured. Regressions were done on Tvol and goiter including age, body surface area, sex, and UI concentration as covariates.
Results: The median UI concentration ranged from 115 µg/L in central Switzerland to 728 µg/L in coastal Hokkaido, Japan. In the entire sample, 31% of children had UI concentrations >300 µg/L, and 11% had UI concentrations >500 µg/L; in coastal Hokkaido, 59% had UI concentrations >500 µg/L, and 39% had UI concentrations >1000 µg/L. In coastal Hokkaido, the mean age- and body surface area–adjusted Tvol was 
2-fold the mean Tvol from the other sites combined (P < 0.0001), and there was a positive correlation between log(UI concentration) and log(Tvol) (r = 0.24, P < 0.0001). In the combined sample, after adjustment for age, sex, and body surface area, log(Tvol) began to rise at a log(UI concentration) >2.7, which, when transformed back to the linear scale, corresponded to a UI concentration of 500 µg/L.
Conclusions: Chronic iodine intakes approximately twice those recommended—indicated by UI concentrations in the range of 300–500 µg/L—do not increase Tvol in children. However, UI concentrations 
500 µg/L are associated with increasing Tvol, which reflects the adverse effects of chronic iodine excess.
Key Words: Thyroid volume • goiter • children • urinary iodine • excess iodine
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