Pregnant women who switch to "healthier" organic milk may be putting the brain development of their unborn babies at risk, experts have claimed.
Milk certified as organic contains about a third less iodine than conventionally-produced milk, according to a new study. The same was found to be true for "ultra-high temperature" (UHT)-processed long-life milk.
Since milk is the primary source of iodine in the UK diet, the discovery is said to have potentially serious health implications.
Iodine is known to be important for the healthy brain development of babies, especially in the early stages of pregnancy.
Previous research has shown that mothers-to-be who are iodine deficient during this critical time can give birth to children with reduced IQs.
Lead scientist Professor Ian Givens, from the University of Reading, 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."
Iodine deficiency was once endemic in parts of the UK, with hundreds of thousands of people in the 1930s and 1940s suffering from goitre.
The condition, caused by a lack of iodine, is marked by abnormal swelling of the thyroid gland in the throat.
In some countries iodine is added to bread or salt, but in the UK supplements given to dairy cows led to milk becoming the primary dietary source of the element, said the researchers.
Experts have described the milk-linked virtual elimination of iodine deficiency in the UK by the 1990s as an "accidental public health triumph".
But 20 years ago doctors warned that changing trends in the production or consumption of milk could affect iodine uptake.
Up to 70% of teenage girls across the UK are now iodine deficient, probably as a result of a decline in milk consumption, said the scientists, whose findings are reported in the journal Food Chemistry.
Catherine Collins, from the British Dietetic Association, said: "This is an interesting piece of research that confirms the lower iodine content of pasteurised organic and UHT-treated milks compared to pasteurised conventional milks produced during winter months.
"These results add to existing data on summer milks, which has also shown a lower level of iodine in organic milks compared to conventional milks.
"Iodine is important as it's an absolutely essential component of thyroxine, the 'master' hormone that controls all metabolic processes, and in particular our metabolic rate.
"Thyroxine cannot be made without sufficient iodine. Low levels of thyroxine are associated with a lower metabolic rate and weight gain, so it's essential for us to have sufficient dietary iodine to maintain metabolic rate at its optimum level.
"Iodine is found in small amounts in plant foods, although the majority of UK dietary iodine comes from milk, seafood and seaweed.
"'One-a-day' multivitamin and mineral supplements usually provide 100% of daily iodine requirements per tablet.
"Milk contributes 40% of our dietary iodine intake, so variations in the levels found in milk can have significant impact on dietary iodine intake.
"The research shows that organic milk provides a third less iodine than conventionally farmed milks, which may be due to differences in the iodine content of winter feeds or different practices in cleaning cow teats prior to milking."
Dr Sarah Bath, Medical Research Council Population Health Scientist Fellow, at the University of Surrey, said: "While there are limitations in this study, the lower iodine concentration in organic milk may have implications for risk of iodine deficiency, as milk and dairy products are the principal source of iodine in the UK diet.
"However, the study has no measures of iodine status in an individual, so we cannot conclude that those who consume organic milk have a lower iodine status, as that depends on the quantity of organic milk consumed and intake of other iodine-rich foods (such as fish)."
Professor Jean Golding, from the University of Bristol, said: "Extremely low levels of iodine in pregnant mothers was shown in the past to be associated with the birth of children with major reductions in cognitive function.
"Milk is a major source of iodine, but it is important to know which sort of milk is most appropriate. This new study has compared different types of milk and shown that, reassuringly, there is no difference in iodine content between full fat, semi-skimmed and skimmed milk.
"Pregnant women should be aware, however, that organic milk appears to have lower levels of iodine in the UK. Perhaps organic farmers could increase the iodine content of their herds with iodine supplementation - meanwhile pregnant women should be aware of the findings of this study."