High levels of brominated flame retardants can alter pregnant women's thyroid hormones, which are critical to a baby's growth and brain development, according to a California study.
The study is considered important because it is the first human research showing a link between the ubiquitous chemicals and altered levels of the hormones in pregnant women. The effects on babies are unknown, but some researchers say it may lead to smaller fetuses, and reduce childen's intelligence and motor skills.
"Normal maternal thyroid hormone levels are essential for normal fetal growth and brain development, so our findings could have significant public health implications," said Jonathan Chevrier, a University of California, Berkeley epidemiologist and lead author of the study, which was published in the journal Environmental Health Perspectives.
The flame-retarding chemicals, polybrominated diphenyl ethers, or PBDEs, are widely found in furniture cushions, carpet pads, electronics and other common household items. They have been detected in about 97 percent of people tested; in the United States, levels are at least 20 times higher than elsewhere.
The Berkeley epidemiologists tested the blood of 270 pregnant women in California's Salinas Valley. The study is part of a larger project monitoring the health of pregnant women and children in the valley, which is predominantly a low-income, Mexican-American farm community.
The women's thyroid-stimulating hormone, or TSH, declined 16.8 percent for every tenfold increase in PBDEs. Low TSH suggests that the thyroid is producing too much hormone on its own. The reductions were found for every type of PBDE.
The high exposures did not lead to clinical hyperthyroidism – a condition involving overproduction of thyroid hormones known to damage fetuses. But their rate of subclinical hyperthyroidism did increase. For every ten-fold increase of PBDEs among the women studied, their risk of subclinical hyperthyroidism doubled.
Scientists do not know whether the thyroid effects were significant enough to harm their fetuses.
Chevier said pregnant women who are clinically hyperthyroid have an increased risk of miscarriage, premature birth and reduced fetal growth. Also, in animal tests, hyperthyroidism in the mothers results in damaged brain development of their offspring. But it is unclear whether subclinical hyperthyroidism – the effect linked to the flame retardants – can damage fetuses.
Research is already in the works to try to address that.
UC Berkeley epidemiologist Brenda Eskenazi, the study's principal researcher, said the team has tested the Salinas children for neurodevelopmental effects such as reduced mental skills, with results expected this summer. In addition, they plan to look for effects on fetal size and weight.
"The thyroid hormone findings we have seen could have an effect on fetal growth and we will be looking for that," she said.
Chevrier said the amounts of flame retardants found in the Salinas women were similar to national averages. As a result, the findings "raise concerns about women exposed to higher levels of PBDEs, which could be made clinically hyperthyroid," he said.
Women in California on average have PBDE levels that are twice as high as the Salinas women. Californians have the highest documented exposures in the world, most likely because the state has the most stringent flammability standards for furniture and bedding.
Ami Zota, an epidemiologist at University of California, San Francisco who did not participate in the Salinas study, said the findings are "important to our understanding of PBDE toxicity in humans as well as to our understanding of thyroid disruption during pregnancy."
She added, though, that other studies should analyze women with higher exposures "because we may see different health effects at these higher levels."
Zota, whose expertise is the effects of environmental factors on reproductive health, added that all pregnant women should be routinely screened for thyroid hormones, which currently is not done. Such screening, she said, "may help detect early assaults of environmental chemicals during pregnancy and create opportunities for intervention and prevention."
Most exposure to the flame retardants comes from household dust, but food also is a major source. The chemicals are slow to break down and build up in fatty tissues, so they accumulate in fish, meat and other foods.
Two of the most ubiquitous PBDEs already have been banned in the United States. But other formulations have taken their place, and little is known about their potential health effects.
One limitation of the study is that it did not analyze the type of PBDE, called deca, which is still in use, largely in electronics. It is being phased out by 2013 in the United States.
Some experts have a theory that the chemicals mimic thyroid hormones, upsetting the balance needed in pregnant women for normal brain development of their children.
Children of New York City mothers with higher PBDE levels had children who scored lower on tests of mental and physical development, according to Columbia University study published last month. And in the Netherlands, children had reduced fine motor skills, lower IQs and more attention problems if their mothers were exposed to higher PBDEs. Similar effects have been documented in animal tests.
An earlier study in Indiana looked for a link between the flame retardants and thyroid hormones in pregnant women, and found no effect. But ithe study was considered too small to be reliable since only nine women were tested. Studies of non-pregnant adults, however, have found similar reductions in TSH in those with high exposures.
The new study detected no changes on the women's thyroxine, or T4, which means they did not have full-blown clinical hyperthyroidism known to harm fetuses. Subclinical hyperthyroidism is when TSH is low but T4 is normal, and it can lead to the clinical condition.
"Although maternal clinical hyperthyroidism has been associated with adverse pregnancy outcomes such as preeclampsia, premature births and low birth weight, few data are available on the direct effects of maternal subclinical hyperthyroidism on fetal and child development," the Berkeley study says. Zota agreed, saying that the impact of moderately low thyroid hormones is "not well understood and should be examined in future research." One large study conducted by University of Texas obstetricians in 2006 found no changes in birth weight or birth defects related to subclinical hyperthyroidism. However, it did not look for latent effects in the children later in life - which the Berkeley team is now doing.
"We have not looked at the health effects of subclinical hyperthyroidism in our cohort yet but…we definitely intend to do that," Chevrier said.
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