An IQ under 85 is considered below average while an IQ measuring anywhere between 85 to 114 is considered average.
In the study, Taylor and colleagues, including those from Bristol University, measured the IQs of 3,123 children aged seven and analysed their DNA.
They were looking for a gene that produces an enzyme called deiodonase-2, which helps to make thyroid hormone available within cells.
The low-IQ variant of this gene produces a faulty version of the enzyme that reduces the availability of thyroid hormone, a leading daily reported.
In children with normal thyroid levels the faulty enzyme has little effect, but in those where the levels are already low the thyroid hormone falls to such low levels that the brain cannot develop properly, researchers found.
They estimate that 4 per cent of all babies in the UK are born each year with the gene plus low thyroid hormone levels.
The discovery raises the prospect of a new test for newborns to spot those with the low-IQ gene variant as well as low hormone levels.
Since both factors are needed to reduce IQ, in theory giving such children thyroid hormone tablets would allow their brains, and IQ, to develop normally, researchers said.


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