Iodine helps brain development, particularly in the womb and first months and improves children's IQ.
Unlike other nations, food in the UK is not fortified with iodine and supplements are not recommended by the NHS.
But experts say pregnant and breastfeeding women should be encouraged to take supplements of iodine.
The findings, published in the journal Lancet Diabetes and Endocrinology, give an economic case for supplements.
Boosting iodine levels of pregnant women would save the NHS £200 per woman in health costs, experts say. And when the child's boost in intelligence is considered - an average 1.22 IQ points - the benefit to society would be £4,500 per child over their lifetime.
With 700,000 births in England and Wales a year, the benefits from a year's newborns over their lifetimes would reach £140million for the NHS and £3.15billion for the economy, with higher productivity and earnings.
In the 1930s and 1940s thousands with iodine deficiency suffered goitre - a swelling of the thyroid gland - but supplements given to dairy cows eradicated the issue. It has reappeared recently, largely due to milk consumption declining.
Study author Professor Kate Jolly, from the University of Birmingham, said, "It's time for all women living in iodine-deficient countries without universal supplementation of iodine, who are pregnant, breastfeeding, or planning a pregnancy to be advised to take a daily supplement containing iodine. Even mild iodine deficiency during pregnancy is associated with children with lower IQs."
Iodine is not produced by the body but the NHS advises that people should be getting all the iodine they need with a varied and balanced diet and does not recommend supplements, even during pregnancy.
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