Scientists found almost seven in ten 14 and 15-year-olds were deficient in the mineral, which is vital for the growth of a baby's brain in the womb, and a lack of it could stunt intelligence.
In Scotland, more than 50 per cent of the schoolgirls tested had iodine deficiency, compared with 85 per cent in Belfast.
Milk is a major source of iodine in the western diet, mainly because it is prevalent in cattle feed.
Iodine deficiency is one of the main preventable causes of mental impairment and studies show if the mother's thyroid is underactive, then the baby can be born with an IQ that is ten to 15 points lower.
Dr Mark Vanderpump, of the Royal Free Hospital in Hampstead, London, and colleagues said milk consumption in the UK has fallen in recent years, and the findings published online by the Lancet suggest this "is responsible for the decline in iodine status".
They analysed urine samples from more than 700 girls and found more than half (51 per cent) had mild iodine deficiency, 16 per cent moderate and 1 per cent severe. They said: "Even mild iodine deficiency could be harmful."
Teenage girls were studied because, as potential mothers-to-be, their iodine levels are seen as the most critical. But the population as a whole is likely to be largely iodine-deficient.
The researchers said: "This study focused on young female participants aged 14-15 years who were pre-pregnancy, because in the short to medium term the children of these young women will be the most susceptible to the adverse effects of iodine deficiency."
The World Health Organisation is recommending that Britain adds iodine to salt, as has been done in Australia and New Zealand, or to the folic acid tablets recommended for women in the first weeks of pregnancy.
Since the 1940s, significant changes in farming practice in the UK have been associated with a rise in the iodine content of milk, particularly during winter months when cattle are dependent on iodine-rich artificial feed.
The researchers recruited girls from secondary schools in Aberdeen, Belfast, Birmingham, Cardiff, Dundee, Exeter, Glasgow, London and Newcastle-upon-Tyne and measured their urinary iodine concentrations.
Higher urinary iodine concentrations were found in the girls in winter, and this seasonal variation may reflect this greater use of iodine-rich winter foodstuffs.
Additionally, successive UK governments from the 1940s encouraged increased milk consumption in schoolchildren. By the 1980s this approach resulted in the iodine content of milk alone being almost sufficient to meet the recommended daily requirement of iodine.
In the study, low milk and high egg intake were linked with iodine deficiency.