05/21/2026
We test for iodine deficiency. It is the number one preventable cause of intellectual disability.
Folate gets the lion's share of attention in prenatal vitamins. Iodine doesn't. Many prenatal products contain little or none of it. A new analysis of 1,211 UK mother-child pairs followed for 15 years shows where exactly that gap shows up.
The Avon Longitudinal Study of Parents and Children (ALSPAC) is one of the largest pregnancy cohorts in the world. In this latest analysis (Keestra et al., European Journal of Nutrition, March 2026), researchers measured urinary iodine-to-creatinine ratios in first-trimester maternal urine and then followed the children for 15 years. At age 15, they administered the Two-Subtest Wechsler Abbreviated Scale of Intelligence (WASI), which produces two separate scores: Vocabulary (verbal intelligence) and Matrix Reasoning (abstract, non-verbal reasoning).
The finding was specific. In linear regression adjusted for confounders, lower first-trimester maternal iodine status predicted lower Vocabulary T-scores at age 15. Matrix Reasoning was not affected. Children of mothers in the severe deficiency group (urinary iodine below 50 µg/g creatinine in the first trimester) scored about 4 T-score points lower on Vocabulary at age 15 than children of mothers in the iodine-sufficient group (150 to 250 µg/g). Full-Scale IQ was also about 3 points lower at severe deficiency, but this is mathematically driven by the Vocabulary component since Matrix Reasoning held steady. The pattern did not differ by s*x.
Why does first-trimester iodine matter specifically? The fetal brain begins building itself before the fetus has a working thyroid gland. The fetal thyroid does not reach functional autonomy until roughly 16 to 20 weeks of gestation. Before that, the developing fetus relies on maternal thyroxine (T4) crossing the placenta to supply the thyroid hormone its neurons need for migration, differentiation, and synapse formation. Maternal thyroid hormone production depends on iodine. If a mother enters pregnancy with insufficient iodine intake, her thyroid cannot produce enough T4 to meet the demand of two organisms in the most metabolically intense window of fetal neurodevelopment.
The most operationally relevant point about timing: the first trimester is when most women don't yet know they're pregnant. By the time a missed period prompts a test, the embryo has already completed neural tube closure and entered the earliest waves of cortical neurogenesis. Whatever iodine status a mother brought into pregnancy is what those processes ran on.
There's a nuance in the paper worth flagging. In iodine-deficient pregnancies, mothers with higher TSH (thyroid-stimulating hormone, the brain's signal telling the thyroid to work harder) had children with higher IQ scores than mothers with normal TSH. The pattern was not seen in iodine-sufficient pregnancies. The interpretation: the maternal hypothalamic-pituitary-thyroid axis appears to partially compensate when iodine is limited, ramping up thyroid drive to maintain T4 output for the fetus. This is consistent with broader pregnancy physiology, where maternal thyroid demand increases roughly 50% by the second trimester. But compensation has limits. Severe deficiency outran what the maternal axis could buffer.
The practical question is what's in a prenatal vitamin. The World Health Organization recommends 250 µg/day of total dietary iodine for pregnant women. The American Thyroid Association recommends at least 150 µg/day from a supplement on top of dietary intake. Many US prenatal vitamins contain 150 µg or less. Some contain none at all. Iodine is not required to be on the label in the same regulatory way that folate is, and brand-to-brand variation is large.
What this looks like in practice. Read the supplement facts panel on whatever prenatal you take and confirm iodine is present at the dose your clinician recommends. Iodine in a supplement should be from potassium iodide, not from a kelp or seaweed extract where the dose varies unpredictably and can fluctuate by orders of magnitude between batches. Iodized salt remains a major dietary source in many countries, but iodized salt use has declined in the US as more households use specialty salts (sea salt, Himalayan, kosher) that are not typically iodized. Dairy is the largest dietary contributor in many Western populations. Saltwater fish and eggs are smaller but reliable.
The most important timing detail. This study measured first-trimester maternal iodine, which means iodine status was already established before pregnancy was confirmed. Iodine intake should be optimized before conception, not after the positive test. If pregnancy is being planned, that means at least the three months prior. If pregnancy is possible, ongoing adequate iodine is the safer default.
What this paper does not say. It does not say that iodine deficiency caused lower vocabulary, only that it predicted lower vocabulary in a large adjusted analysis. It does not address whether postnatal iodine intervention would change cognitive trajectory, because the relevant exposure window is already past at birth. It does not say that mild deficiency (50 to 149 µg/g) is harmful, since the effect was only statistically significant at the severe threshold in the categorical analysis.
What it does say is that what a mother's prenatal contained 15 years ago is measurable in her child's verbal intelligence today. That's a long horizon for one nutrient.
Citation: Keestra SM, Königs M, van Welie N, Dreyer K, Oosterlaan J. Iodine deficiency in the first pregnancy trimester and intelligence in adolescence. European Journal of Nutrition. 2026 Mar.
ALSPAC cohort, n = 1,211 mother-child pairs, first-trimester urinary iodine-to-creatinine ratios, Two-Subtest WASI at age 15.
