THURSDAY, Oct. 4 (HealthDay News) -- New research finds that BPA -- a chemical widely used in the manufacture of hard plastics, cans and even store receipts -- is associated with lower levels of thyroid hormone in both pregnant women and their newborn boys.

At this point, the implications of the findings to human health are unclear, although abnormal thyroid hormone levels have been associated with changes in thinking skills, behavior and growth, according to the research, which was published online Oct. 4 in Environmental Health Perspectives.

This is the first study to show an association between BPA (or bisphenol A) and thyroid hormone in pregnant women, according to the study's lead author, Kim Harley, an adjunct associate professor of public health at the University of California, Berkeley.

Almost all -- 95 percent -- of women of reproductive age have enough BPA in their urine to be detectable. BPA has also been found in placental tissue and amniotic fluid and is known to have estrogen-like activity.

There has been concern that BPA may be linked with reproductive and developmental problems and, in July, the U.S. Food and Drug Administration banned BPA in baby bottles and sippy cups.

In the new study, researchers measured BPA levels in the urine of 335 pregnant women, once during the first half of pregnancy and once in the second half.

They also measured levels of thyroid hormones in the women and their newborn babies.

Although thyroid hormone levels fell within a normal range, higher levels of BPA exposure were associated with lower thyroid levels in both mothers and newborn boys, though not girls, the investigators found.

It's not clear why the gender difference but one hypothesis, Harley said, "is that boys may be less able to detoxify BPA than girls."

The hormone changes seen in the mothers weren't overly dramatic, but the exposure to the fetus could be another story entirely, an expert suggests.

"We have increasing concern about endocrine-disrupting chemicals," said Dr. Keith Cryar, an endocrinologist with Scott & White in Temple and in Round Rock, Texas. "We know that very small differences can sometimes have profound effects on the developing fetus."

The researchers also found that the effects of BPA on thyroid hormone were transient.

"It's good news in the sense that, if we were able to stop exposure to BPA, there might not be a long-term effect on the human body," said another lead author, Jonathan Chevrier, a research epidemiologist at UC Berkeley's Center for Environmental Research and Children's Health. "On the other hand, what we're experiencing right now is that people are being continuously exposed to BPA so it's not necessarily good news in the present state of things."

Because the study was limited to poor immigrant Mexican-American women and their babies, the authors pointed out that the findings cannot necessarily be generalized to other populations.

Nor can the study prove that BPA causes declines in thyroid hormone levels.

But the findings do provide some revelations about BPA.

"BPA has gotten a lot of attention for being an estrogen, but it's more complicated than that," Harley said. "We know from laboratory studies that it interferes with other hormones, including thyroid hormone."

The findings, said Robin Dodson, a research scientist with the Silent Spring Institute in Newton, Mass., "are important to understand the health effects of BPA."

And the good news is that there are ways to limit exposure to BPA, including adding fresh foods to your diet while avoiding packaged foods, Dodson added.

Harley said this is the first of several studies on BPA scheduled to come out in the coming months. The next two studies will look at the effects of BPA on neurodevelopment in children and on obesity.

More information

For more on BPA, visit the Mayo Clinic.