Pregnant women who don't produce enough thyroid hormone are nearly four times likelier to give birth to autistic children than their healthy peers, a new study has claimed.
Scientists from the Houston Methodist Neurological Institute in US and Erasmus Medical Centre in Rotterdam, Netherlands studied more than 4,000 Dutch mothers and their children.
Their results support the growing view that autism spectrum disorders can be caused by a lack of maternal thyroid hormone, which past studies have shown is crucial to the migration of foetal brain cells during embryo development.
"It is increasingly apparent to us that autism is caused by environmental factors in most cases, not by genetics. That gives me hope that prevention is possible," said lead author Gustavo Roman, a neurologist and neuroepidemiologist who directs the Nantz National Alzheimer Center.
The researchers also found that autistic children had more pronounced symptoms if their mothers were severely deficient for T4, also called thyroxine. Mild T4 deficiencies in mothers produced an insignificant increase in autistic children's symptoms.
The most common cause of thyroid hormone deficiency is a lack of dietary iodine - because both the thyroid hormones, T3 and T4, contain that element.
The present work was based on the Generation R Study, conducted by Erasmus Medical Centre doctors and social scientists, in which thousands of pregnant women were voluntarily enrolled between 2002 and 2006.
Researchers identified 80 "probable autistic children" from a population of 4,039.
Around 159 mothers were identified as being severely T4 deficient (defined as having 5 per cent or less of normal T4, but producing a normal amount of thyroid stimulating hormone), and 136 were identified as mildly T4 deficient.
The researchers found a weak association between mild T4 deficiency and the likelihood of producing an autistic child, but a strong association between severe T4 deficiency and autism (3.89 more likely, as compared with mothers with normal thyroid hormone).
The study presents a troubling correlation, but it does not prove that the thyroid function of expecting mothers causes autism in their children, researchers said.
The study will be published in the journal Annals of Neurology.
