Study leader Margaret Rayman of the University of Surrey and colleagues at the University of Bristol analyzed mother/child pairs from the Avon Longitudinal Study of Parents and Children study by measuring urinary iodine concentration -- and creatinine to correct for urine volume -- in stored samples from 1,040 first-trimester pregnant women.
They also tested the IQ of the offspring at age 8 and reading ability at age 9.
The study said two-thirds of the pregnant women tested and were found to be iodine deficient.
The study, published in The Lancet, said children of mothers with the lowest levels of iodine were more likely to have children with lower IQ and lower levels of reading ability.
"Our results show the importance of adequate iodine status during early gestation and emphasize the risk that iodine deficiency could pose to the developing infant, even in a country classified as only mildly iodine deficient," the researchers concluded. "Iodine deficiency in pregnant women in Britain should be treated as an important public health issue that needs attention."