University of Reading scientists found that milk certified as 'organic', as well as conventional long-life milk treated at ultra-high temperatures (UHT), was a third lower in iodine than conventionally-produced fresh milk.
Researchers from Reading's Food Production and Quality Division said the findings had potentially serious public health implications, as most iodine in our diet comes from milk products.
Iodine is particularly crucial for the brain development of babies, particularly in the early stages of pregnancy. Studies have found that iodine deficiency in mothers during these stages can lead to children with a lower IQ.
Professor Ian Givens, University of Reading, who led the research, said: "People are increasingly buying organic and UHT milk for perceived health benefits or convenience. But our research shows that this trend could have serious implications for public health.
"Iodine deficiency ought to be a health problem from the past. But unless this situation is carefully monitored, we risk sleepwalking into a new health crisis in the 21st century.
"Organic and UHT milk is not bad for you, and drinking all types of milk has numerous health benefits. But to get the same amount of iodine as in a pint of conventional pasteurised milk, you would need to drink around an extra half-pint of organic or UHT milk."
A previous study has shown that some organic milk, produced in summer when organic dairy herds are fed almost exclusively on grass, was lower in iodine. This latest study shows that the trend is year-round, as well as making an assessment of iodine content in UHT milk for the first time.