Iodine supplementation improves cognition in mildly iodine-deficient children

184 children aged 10-13

Children in Dunedin, New Zealand - a HIGH-INCOME country - who were MILDLY iodine deficient at baseline (median urinary iodine 63 ug/L, thyroglobulin 16.4 ug/L). This is the crucial intermediate case - a well-fed rich-country population carrying a real, biochemically documented micronutrient shortfall.

Randomized, placebo-controlled, double-blind trial. 150 ug iodine daily or placebo for 28 weeks. Cognition assessed with four subtests of the Wechsler Intelligence Scale for Children - an independent standardized instrument, not a bespoke battery. Biochemical verification of the deficiency at baseline and of repletion at follow-up, so the mechanism is documented rather than assumed. Single trial, n = 184, so grade B and capped at confidence medium on its own.

Iodine status improved in the supplemented group (urinary iodine 63 to 145 ug/L, thyroglobulin 16.4 to 8.5) while the placebo group remained deficient (81 ug/L, 11.6). Two of four cognitive subtests improved significantly - picture concepts (P = 0.023) and matrix reasoning (P = 0.040) - and two did not: letter-number sequencing (P = 0.480) and symbol search (P = 0.608). Overall cognitive score was 0.19 SD higher in the iodine group (P = 0.011). The gains are in perceptual reasoning specifically. Note that two of four subtests moving is the same pattern that would be called "subgroup mining" in a less biologically grounded trial; the documented biochemical repletion is what makes this credible.

None - double-blind placebo-controlled randomization with biochemical verification of the mechanism.