High levels of common flame-retardant chemicals appear to alter thyroid hormone levels in pregnant women, according to a new study. The chemicals, called PBDEs, or polybrominated diphenyl ethers, are known to interfere with blood chemistry in animals, but this is the first large study to investigate levels of PBDEs and a sensitive thyroid hormone in pregnant women.
The link is important because the chemicals, in high enough levels, could affect the pregnancy and the health of the fetus, said one of the study's authors, Jonathan Chevrier, a researcher in epidemiology and in environmental health sciences at UC Berkeley.
"Normal maternal thyroid hormone levels are essential for normal fetal growth and brain development, so our findings could have significant public health implications," Chevrier said in a news release. "These results suggest that a closer examination between PBDEs and these outcomes is needed.”
The study was released Monday in the journal Environmental Health Perspectives.
PBDEs are found in carpets, textiles, foam furnishings, electronics and plastics. U.S. fire safety standards implemented in the 1970s led to increased use of PBDEs, which can leach out into the environment and accumulate in human fat cells. PBDEs can be found in the blood of most American residents, and levels are especially high among California residents because of the state's rigorous flammability laws. Concentrations of the chemicals in blood and breast milk have increased dramatically in the last three decades.
The researchers analyzed blood samples from 270 women taken around the end of their second trimester of pregnancy. The researchers measured concentrations of 10 PBDE chemicals, two types of thyroxine (T4) and thyroid-stimulating hormone (TSH). TSH is a substance that drops when the thyroid gland is producing too much hormone. The study also controlled for such factors as maternal smoking, alcohol and drug use, and exposure to lead and pesticides.
The researchers found that a 10-fold increase in each of the PBDE chemicals was associated with decreases in TSH ranging from 10.9% to 18.7%. At these levels, the women met the definition of subclinical hyperthyroidism, which indicates an early stage of thyroid malfunction.
"Low TSH and normal T4 levels are an indication of subclinical hyperthyroidism, which is often the first step leading toward clinical hyperthyroidism," Chevrier said. "Though the health effect of subclinical hyperthyroidism during pregnancy is not well understood, maternal clinical hyperthyroidism is linked to altered fetal neurodevelopment, increased risk of miscarriage, premature birth and intrauterine growth retardation."
It's not known just how the chemicals interfere with thyroid function. PBDEs could bind to thyroid receptors and alter how the hormone is released.
A study released earlier this year from the same research group found that women with higher exposures to flame retardants took longer to get pregnant.
"Our results suggest that exposure to PBDE flame retardants may have unanticipated human health risks," said Brenda Eskenazi, a professor of epidemiology and of maternal and child health at UC Berkeley.
-- Shari Roan
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