The finding raises the possibility of treating children born with the variant with synthetic thyroid hormone to help enable normal intellectual development.
'If other studies confirm our finding then there may be benefit in carrying out a genetic test for this variant in addition to the standard neonatal thyroid screening which would identify children most at risk of developing low IQ', said lead researcher Dr Peter Taylor from Cardiff University.
In their study, presented at a conference of the Society for Endocrinology in the UK, the researchers used information from thyroid function tests from over 3,000 children taken when they were seven years old and IQ tests when they were eight years old. They investigated the influence of a gene variant for an enzyme known as deiodinase-2, which turns thyroid hormone from its inactive to its active state.
They found that children with this gene variant, which made the enzyme less effective at its job, and lower levels of thyroid hormone were four times more likely to have an IQ of 85 or lower. In the general population one in six people have an IQ below that threshold and the average IQ is 100.
About four percent of children had both the gene variant and lower thyroid hormone levels. Children with the gene variant and normal thyroid hormone levels had average IQ scores. This, Dr Taylor says, 'raises the possibility that children at risk could be treated with standard thyroid hormone tablets to compensate for impaired thyroid hormone processing'.
However, any prevention programme would need to be trialled. Newborn babies already have their thyroid hormone levels tested and an easy gene test would need to be developed and added.
Although low thyroid hormone, the gene variant and IQ are associated when children are seven years old, it remains to be seen whether those associations hold when thyroid is measured earlier in life and whether giving synthetic thyroid hormone does improve IQ .
The children were part of the Children of the 90s study, which has been looking at children and their families from the Bristol area born in the early 1990s.