Pregnant women who switch to "healthier" organic milk may be inadvertently putting the 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.
Milk is a major source of iodine for many women, so 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. Its deficiency is the most preventable cause of mental retardation in the world.
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 [473ml] of conventional pasteurised milk, you would need to drink around an extra half-pint of organic or UHT milk."
Iodine deficiency was once endemic, 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.
As in the UK, Australia's iodine deficiency rate has had an increase in recent years. This is in line with changes in the dairy industry and a reduction in the consumption of iodised salt.
In Australia, bakers are required to replace regular salt with iodised salt in bread. According to Nutrition Australia, "prepackaged breads must list 'iodised salt' as an ingredient on product labels, however, unpackaged bread does not require such labelling".