To determine the prevalence of iodine deficiency, its causes and its association with intelligence quotient (IQ) in Mexican schoolchildren.

Cross-sectional analytical study, in which determinations of thyroid gland size, urinary iodine excretion, IQ, iron nutritional status, physical anthropometry, family consumption of goitrogenic foods, type/origin and iodine saturation of salt consumed at home and coliform organisms in drinking water were performed, and the association of each variable with IQ scores was evaluated by multiple regression analyses.

Municipality of Cuauhtémoc, in Colima, Mexico (altitude: 600–2700 m above sea level). Sea salt is extracted manually nearby and often used for human consumption. Goitre remains present in the region despite over half a century of mandatory salt iodination in the country.

Three hundred and three children, similar proportions of boys and girls, mean age 9·3 years, randomly selected from 19 public elementary schools.

Overall goitre rate was 21·4 %; low urinary iodine excretion was found in 19·5 % of the children, high urinary iodine excretion in 32·0 %. IQ scores were transformed into percentile values, with the following categorisation: ≤P5 (low IQ), 48·5 %; >P5 to ≤P25 (below average), 24·2 %; >P25 to <P75 (average), 18·8 %; ≥P75 to <P95 (above average), 3·6 %; ≥P95 (high IQ), 4·9 %. Ninety-two per cent of the population used iodinated salt, but deficient iodine saturation (<50 ppm) was found in 86·8 % of salt samples. The main goitrogenic foods consumed were peanuts (by 31·5 % of the sample), cabbage (30·1 %), broccoli (27·7 %) and cauliflower (25·7 %). Median counts of coliform organisms (colony-forming units/100 ml of drinking water) were: 207·5 (well water), 151 (cisterns), 52 (private homes), 25 (elementary schools) and 12 (kindergartens). Moderate iodine deficiency was associated (P < 0·05) with a 4·26 times higher risk of low IQ.

There is a perturbing negative impact of these findings on human capital acquisition for the region and the country. More attention is needed to ensure effective salt iodination processes, particularly in regions where goitrogens may contribute to the negative effects of iodine deficiency on the intellectual development of children.