"Even with multiple measurements, you have to start out realizing that establishing these links is enormously challenging," he said.
Results from previous studies investigating BPA and thyroid function have been inconsistent. Different studies on rats have shown BPA to increase, decrease and have no effect on thyroid hormones after prenatal exposure. In human studies – before now only conducted on adults and teens – BPA has been linked to both increased and decreased thyroid hormones.
The new study found significant decreases in thyroid hormones in the babies' blood only when compared with the women’s BPA levels shortly before they gave birth, not during their first and second trimesters. That may mean that BPA’s effect on the fetuses’ hormones is temporary or comes only at certain times.
“This association was strongest when BPA was measured in the third trimester of pregnancy, which may either be due to a transient effect of BPA on thyroid-stimulating hormone or a developmental window of susceptibility,” the study said.
The mothers also had reductions in some thyroid hormones that correlated to their BPA levels. However, the type of hormone, called T4, "is not biologically active" so the importance is unknown, the researchers wrote. The study controlled for their iodine intake, which can affect thyroid hormone levels, and for other chemicals they were exposed to.
The researchers, led by epidemiologists Brenda Eskenazi and Kim Harley, have tracked this group of Salinas Valley women and their children since before they were born in 1999 and 2000. They are investigating an array of chemicals, particularly pesticides, to investigate whether the children’s health is at risk.
The children tested as newborns for thyroid hormones in this study are now teenagers, and Chevrier and his colleagues plan to look for any potential effects of the lower hormones. "We are aware of no studies that investigated the developmental effect of lower but normal neonatal TSH," they wrote.
Only a few studies have investigated the possible neurological effects of BPA in children, including two that found a link to hyperactivity in girls and other behavior problems. Animal studies have found impaired memory, gene changes and altered behavior.
Cheryl Stein, an assistant professor at the Mount Sinai School of Medicine, said usually it is high thyroid hormone levels that are cause for concern because they can spur development problems or mental retardation. This is because higher levels indicate the body is trying to make up for a thyroid that isn’t functioning properly.
But too little thyroid hormone also can be problematic, causing reduced IQs or learning problems, Zoeller said.
Scientists in the Netherlands studied 220 women and reported in 1999 that those with the lowest thyroid hormone levels had babies who scored lower on mental and motor skill tests. They warned that it “may be an important risk factor for impaired infant development.”
Zoeller, who was not involved with the new study, said the potential health impacts remain vague because it measured hormones in the babies’ blood instead of tissue, which gives a more complete picture of their functioning.
“This should really be a trigger to look further into this relationship,” Zoeller said.
The idea that “many lifelong chronic diseases start in the womb” has gained wide support in recent years, according to a 2012 study from the Center for Research on Occupational and Environmental Toxicology. And “pretty much anything mom is exposed to the fetus is exposed to,” Vandenberg said.