Pregnant women with thyroid issues are nearly 4 times more likely to produce a autistic child, a new study has suggested.
The link emerged from a study of more than 4,000 Dutch mothers and their children, and it supports a 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.
Lead author Gustavo Roman, M.D., a neurologist and neuroepidemiologist who directs the Nantz National Alzheimer Center, said that it's increasingly apparent to us that autism is caused by environmental factors in most cases, not by genetics.
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 (Rotterdam, Netherlands) doctors and social scientists, in which thousands of pregnant women were voluntarily enrolled between 2002 and 2006.
Blood was withdrawn from the mothers at or around 13 weeks into their pregnancies to measure levels of T4 and two proteins that could indicate the cause of thyroid deficiency.
Six years later, mothers were asked to describe the behavioral and emotional characteristics of their children using a standardized psychology checklist.
Researchers identified 80 "probable autistic children" from a population of 4,039 -- a number consistent with the Dutch rate of autism spectrum disorders. 159 mothers were identified as being severely T4 deficient (defined as having 5 percent 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 has been published in The Annals of Neurology.