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Background: Iodine deficiency early in the life cycle-the "first 1000 days"-can cause hypothyroidism and irreversibly impair neuromotor development. However, the relative vulnerability among women and infants during this critical period is unclear, making it difficult for country-based programs with limited resources to prioritize their iodine interventions.Objective: Our aim was to determine the prevalence of thyroid hypofunction in women and infants living in an area of moderate-to-severe iodine deficiency.Methods: In a cross-sectional survey in Morocco, we measured urinary iodine concentrations (UICs) and concentrations of thyroid-stimulating hormone (TSH) and total or free thyroxine (TT4 or fT4, respectively) in women of reproductive age (n = 156), pregnant women (n = 245), and lactating women (n = 239) and their young infants (n = 239). We calculated daily iodine intakes and measured iodine concentrations in breast milk and household salt. We compared the incidence of hypothyroidism between the 3 groups of women and with the infants.Results: Women of reproductive age, pregnant women, and lactating women had median (IQR) UICs of 41 (29-63), 32 (17-58), and 35 (19-62) μg/L; and estimated iodine intakes were ∼60%, 22%, and 26% of Recommended Nutrient Intakes (RNIs). The infants' median UIC was 73 (28-157) μg/L, which was greater than for all 3 groups of women (P < 0.001), and their dietary intakes were 27% of the RNI. The prevalence of hypothyroidism was not significantly different between the 4 groups, whereas the prevalence of hypothyroxinemia was higher in infants (40%) than in the 3 groups of women (11-14%) (P < 0.001). The median breast-milk iodine concentration was 42 (26-81) μg/L. Only 6% of salt samples were adequately iodized to a concentration of ≥15 ppm; 54% were inadequately iodized and 40% contained no measurable iodine.Conclusions: In an area of moderate-to-severe iodine deficiency, the prevalence of thyroid hypofunction is ∼4-fold higher in young infants compared with the 3 groups of women, suggesting that, in the "first 1000 days," infants are more vulnerable than their mothers and that programs should prioritize iodine prophylaxis for this group.