Context: The median urinary iodine concentration (UIC) is a biomarker of iodine intake. According to the World Health Organization, a median UIC in the range 100–199 μg/L indicates adequate and 200–299 μg/L more than adequate intake. Thyroglobulin (Tg) may be a promising functional biomarker of both iodine deficiency and excess. Objectives: Using a standardized dried blood spots-Tg assay in children, we evaluated the Tg response to both low- and high-iodine intake and estimated the population cutoff point for iodine deficiency or excess. Also, we compared thyroid functions within the UIC ranges of 100–199 vs 200–299 μg/L. Design and Setting: We conducted a cross-sectional study in primary schools in 12 countries. Subjects: Subjects were 6 to 12 years old (n = 2512). Main Outcome Measures: We measured UIC, TSH, total T4, Tg, and thyroid antibodies. Results: Over a range of iodine intakes from severely deficient to excessive, Tg concentrations showed a clear U-shaped curve. Compared with iodine-sufficient children, there was a significantly higher prevalence of elevated Tg values in children with iodine deficiency (UIC <100 μg/L) and iodine excess (UIC >300 μg/L). There was no significant change in the prevalence of elevated Tg, TSH, T4, or thyroid antibodies comparing children within the UIC ranges of 100–199 vs 200–299 μg/L. Conclusions: In school-aged children, 1) Tg is a sensitive indicator of both low and excess iodine intake; 2) a median Tg of <13 μg/L and/or <3% of Tg values >40 μg/L indicates iodine sufficiency in the population; 3) the acceptable range of median UIC in monitoring iodized salt programs could be widened to a single category of sufficient iodine intake from 100 to 299 μg/L.