Researchers writing in the Lancet medical journal found two-thirds of teenage girls are deficient in the mineral, and blames not drinking enough milk for the low levels.
Lack of iodine in pregnancy can lead to problems in babies at birth, and researchers reckon even 'mild' levels of deficiency can cause problems.
The study looked at girls aged 14-15 from nine different schools across the UK, analysing samples from more than 700 girls.
Half had mild iodine deficiency, and 16 per cent moderate, and one per cent severe deficiency.
Dr Mark Vanderpump, who led the researchers, said the impact of iodine deficiency in pregnancy could not be under-estimated:
'Mild iodine deficiency impairs cognition in children, and moderate to severe deficiency in a population reduces IQ by 10-15 points.'
Experts are now calling for iodine to be added to salt like in other countries, or to folic acid supplements which are recommended during the early stages of pregnancy.