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Background: Optimal iodine intake during infancy is critical for brain development, but no estimated average requirement (EAR) is available for this age group. Objective: We measured daily iodine intake, excretion, and retention over a range of iodine intakes in early infancy to determine the minimum daily intake required to achieve iodine balance. Design: In a dose-response crossover study, we randomly assigned healthy infants (n = 11; mean ± SD age 13 ± 3 wk) to sequentially consume over 33 d 3 infant formula milks (IFMs) containing 10.5, 19.3, and 38.5 µg I/100 kcal, respectively. Each IFM was consumed for 11 d, consisting of a 6-d run-in period followed by a 4-d balance period and 1 run-out day. Results: Iodine intake (mean ± SD: 54.6 ± 8.1, 142.3 ± 23.1, and 268.4 ± 32.6 µg/d), excretion (55.9 ± 8.6, 121.9 ± 21.7, and 228.7 6 39.3 µg/d), and retention (-1.6 ± 8.3, 20.6 ± 21.6, and 39.8 6 34.3 µg/d) differed among the low, middle, and high iodine IFM groups (P, 0.001 for all). There was a linear relation between daily iodine intake and both daily iodine excretion and daily iodine retention. Zero balance (iodine intake = iodine excretion, iodine retention = 0 µg/d) was achieved at a daily iodine intake of 70 µg (95% CI: 60, 80 µg). Conclusion: Our data indicate the iodine requirement in 2- to 5-mo-old infants is 70 µg/d. Adding an allowance for accumulation of thyroidal iodine stores would produce an EAR of 72 µg and a recommended dietary allowance of 80 µg. This trial was registered at clinicaltrials.gov as NCT02045784. [ABSTRACT FROM AUTHOR]