A genetic test could identify children with impaired thyroid function at risk of having low IQ, research suggests.
Children under seven with a common gene variant had a four-fold greater risk of developing an IQ under 85 if they also had reduced levels of thyroid hormone, scientists found.
The discovery raises the possibility of screening children for the problem and treating them with thyroid hormone pills.
Normal thyroid hormone levels are known to be essential for brain development in childhood.
The research focused on an enzyme called deiodinase-2 that is involved in processing thyroid hormones within cells.
A mutation in the gene coding for the enzyme has been associated with other health problems including diabetes and high blood pressure.
In the new study, scientists from the universities of Cardiff and Bristol looked at genetic data of 3123 children under the age of seven who also had their IQs tested.
Those with thyroid hormone levels at the bottom of the normal range who also possessed the deiodinase-2 variant were four times more likely to have an IQ under 85.
Children with lower thyroid hormone levels alone were not at greater risk of low IQ.
"If other studies confirm our finding then there may be benefit in carrying out a genetic test for this gene variant in addition to the standard neonatal thyroid screening, which would identify children most at risk of developing low IQ," lead researcher Peter Taylor, from the University of Cardiff's School of Medicine, said.
"Children with satisfactory thyroid hormone levels, together with the genetic variant, have normal IQ levels, which raises the possibility that children at risk could be treated with standard thyroid hormone tablets to compensate for impaired thyroid hormone processing."
The findings were presented at the Society for Endocrinology's British Endocrine Societies conference in Liverpool.
