Schoolgirls have dangerously low levels of iodine, which could put the health of future generations at risk, claim British researchers. Two-thirds of teenagers are deficient in the trace mineral, says a new study, partly because consumption of milk has plummeted in recent years. But lack of iodine in pregnancy can lead to mental retardation in babies, with researchers saying even mild levels of deficiency can be harmful. Experts are calling for iodine to be added to salt as already happens in some countries or to folic acid supplements routinely recommended during the early stages of pregnancy. The study is the latest to warn of a growing number of young and pregnant women who may jeopardise the future health of their babies by not eating a balanced diet, or taking additional vitamins and minerals. A new study in The Lancet medical journal looked at girls aged 14-15 years from nine schools throughout the United Kingdom. Researchers analysed urine samples from more than 700 girls and found two-thirds were deficient in iodine. Altogether, half had mild iodine deficiency, a further 16 per cent had moderate and one per cent severe deficiency. They studied teenage girls because young women are most liable to see the ill-effects if they get pregnant. However, the study concluded the UK population as a whole is now iodine deficient. Dr Mark Vanderpump, who led the researchers, said the potential impact of iodine deficiency in pregnancy could not be under-estimated. He added: Mild iodine deficiency impairs cognition in children, and moderate to severe deficiency in a population reduces IQ by 10-15 points. Adding iodine to salt, as happens in countries like the United States and Switzerland, remains the most cost-effective way to control iodine deficiency says the study. Iodine is essential for the body, and in pregnancy. It is a trace mineral that helps make the thyroid hormones, which keep cells and the metabolic rate healthy. Deficiency has been linked with a range of disorders, including thyroid problems such as swelling, as well as cancer and heart disease. Pregnant women who are deficient run the risk of miscarriage, stillbirth and congenital abnormalities. However, too much iodine can cause overactivity of the thyroid.