Although one glass a day is enough to meet 50 per cent of the daily requirement, falling consumption is thought to be the reason that rising numbers of Britons are now deficient.
Even mild iodine deficiency in pregnant women can damage the mental abilities of a baby in the womb and, according to the report, moderate to severe iodine deficiency can reduce IQ by 10 to 15 points.
The thyroid gland, which produces hormones that regulate metabolism and fertility, also needs iodine to function.
Low levels of thyroid hormones can lead to women becoming infertile.
According to the US National Institutes of Health, iodine deficiency may also be linked to an increased risk of breast, thyroid, ovarian and prostate cancers.
A new report says that half of women of child-bearing age may also suffer with iodine deficiency.
‘These new findings have major importance,’ says Dr Mark Vanderpump, consultant endocrinologist at the Royal Free Hospital in North London, who carried out the research.
‘The body cannot make iodine – we need to get it from diet, but there is very little in food, unless it has been added, and many countries have acted to add it to salt.
Changes in farming practice in the UK, dating back to the Forties, led to a rise in the iodine content of milk because cattle were (and still are) given iodine-rich artificial feed.
At the same time, successive governments encouraged increased milk consumption in schoolchildren.
The combination of these two factors meant that by the Eighties, the iodine content of milk was enough to meet daily requirements.
However, later that decade, free school milk was ended and research shows that since then there has been a steady decline in iodine levels.
‘It is likely to be the reduced amount of milk now drunk since the Eighties that is responsible for the resurgence of iodine deficiency,’ says Dr Vanderpump.
Failing a return to free school milk, researchers favour adding iodine to salt, which they say would be safe and cost-effective.