Iodine: the quiet nutrient that shaped a century of public health

Iodine is a trace element the body needs to make thyroid hormone, and getting too little — especially early in life and in pregnancy — is the most common preventable cause of impaired brain development in the world [s2]. Severe deficiency was largely conquered by adding iodine to salt, one of public health's great cheap wins, but mild deficiency has re-emerged in some wealthy countries among pregnant women and people who avoid dairy and iodised salt [s2][s3].

Why iodine matters

Iodine's entire importance runs through the thyroid. Without enough of it, the gland cannot make the hormones that regulate metabolism and, critically, guide the developing brain [s2][s4]. In severe deficiency the consequences are stark — goitre, hypothyroidism, and in the worst cases irreversible intellectual disability in children exposed in the womb [s2]. As of the most-cited global estimate, about 2 billion people worldwide had insufficient iodine intake, with South Asia and sub-Saharan Africa worst affected, and roughly a third of the world's population still lived in iodine-deficient areas [s2].

The remedy is unusually simple and cheap. In nearly all countries the best strategy to control iodine deficiency is iodisation of salt, which the review describes as one of the most cost-effective ways to contribute to economic and social development; where salt iodisation is not feasible, supplements can be targeted at susceptible groups [s2]. That single intervention is why iodine deficiency has receded from a mass disabler to, in many places, a monitoring problem.

How iodine status is measured

Because iodine is excreted in urine, status is judged at the population level by median urinary iodine concentration rather than by blood tests [s1]. For school-age children, WHO defines an adequate population median as 100 to 199 micrograms per litre; below 100 the population's intake is insufficient, graded mild at 50 to 99, moderate at 20 to 49, and severe below 20 [s1]. Pregnancy raises the requirement: for pregnant women the median should sit between 150 and 249 micrograms per litre, with values below 150 counted as insufficient [s1]. These are population yardsticks, not personal diagnostic thresholds — a single person's urine iodine swings too much from meal to meal to be a reliable individual test [s1].

The re-emergence in pregnancy

The most influential modern evidence that mild deficiency still matters came from the UK. Researchers analysing the Avon Longitudinal Study of Parents and Children (ALSPAC) measured iodine in stored first-trimester urine from 1,040 pregnant women and classified the group as mildly-to-moderately deficient, with a median urinary iodine concentration of 91.1 micrograms per litre [s3]. After adjusting for 21 possible confounders, children of women with an iodine-to-creatinine ratio below the WHO cut-off of 150 micrograms per gram were more likely to score in the lowest quartile for verbal IQ (odds ratio 1.58, 95% confidence interval 1.09 to 2.30), reading accuracy (1.69, 1.15 to 2.49) and reading comprehension (1.54, 1.06 to 2.23) at ages 8 and 9 [s3]. Scores worsened steadily as maternal iodine fell [s3]. It was an observational study, so it cannot prove causation on its own, but it fits the well-established biology of iodine and fetal brain development [s2][s3].

The adult twist: more is not always better

Iodine is one of the few nutrients where both too little and too much cause thyroid disease, so correcting deficiency has to be done carefully. In mild-to-moderate deficiency the thyroid compensates by working harder, but chronic overstimulation raises the population prevalence of toxic nodular goitre and hyperthyroidism [s4]. Introducing iodised salt to a long-deficient population can transiently increase some thyroid disorders before rates settle, and pushing intake too high brings its own small risks — which is why WHO flags population medians of 250 to 499 micrograms per litre as above requirements and 500 or more as excessive [s4][s1]. The consensus, though, is unambiguous: the small risks of iodine excess are far outweighed by the substantial harms of deficiency [s2].

Everyday iodine comes largely from dairy, seafood, eggs and iodised salt, which is why intake can fall in people who cut those out. The trade-offs of reducing salt for other reasons are covered in our review of how much salt the evidence supports; the nutrient gaps that can open up on plant-based eating, iodine among them, in vegan diets and nutrient gaps; dairy's place in the diet in dairy and cardiometabolic health; and another pregnancy-nutrition question in omega-3 and preterm birth.

How to read this

The evidence here is some of the most solid in nutrition: the biology is understood, the population data are consistent, and the intervention — iodised salt — has a track record measured in decades [s2][s4]. The live issue is not whether iodine matters but whether monitoring has slipped, letting mild deficiency return in groups that avoid its main dietary sources, pregnant women most consequentially [s3]. This article is informational and not dietary or medical advice; questions about iodine intake in pregnancy should be discussed with a qualified clinician.