Mild iodine deficiency during pregnancy could be dimming the intellect of some babies born in the UK, say researchers.
Their study of 1,000 families, published in the Lancet, showed lower IQs and reading scores in primary school pupils whose mother had had too little iodine while pregnant.
Academics advise women of child-bearing age to maintain iodine in their diets by eating dairy products and fish.
Women were warned not to take seaweed pills, as they contain too much iodine.
Iodine is essential for the development of the brain as it is needed to build some of the body's hormones. A severe deficiency is the leading cause of preventable brain damage in the world.
It was mainly thought of as a problem in developing countries, yet previous studies have also suggested that some women in the UK are mildly deficient. The impact of low-level deficiency was, however, previously unknown.
Researchers at Surrey and Bristol Universities looked at iodine levels in urine samples taken from pregnant women in south-west England.
The study showed that iodine deficiency was common - affecting two-thirds of women.
Their children went on to have slightly lower IQs at the age of eight and worse reading ability aged nine.
Dr Sarah Bath told the BBC: "We saw a three-point IQ difference between children who were born to mothers with low iodine in early pregnancy and children who were born to mothers above the cut-off."
The researchers said this "may prevent a child reaching their full potential" and was an "important public health issue".