Low levels of exposure to polychlorinated biphenyls ( PCBs ), certain pesticides, and mercury can interfere with thyroid hormone levels during pregnancy.

The study focuses on the impact of low environmental exposure to these toxicants during pregnancy.

Though thyroid status is monitored during pregnancy, testing is generally limited to measurement of TSH ( thyroid-stimulating hormone ).
Variation in the levels of T3 and T4 ( hormones secreted by the thyroid that impact metabolism ) has not traditionally been assessed, despite the findings of a previous study that linked low T3 levels with postpartum depression.

In the current study, researchers recruited volunteers during their first prenatal clinic visit and monitored lifestyle habits and thyroid function throughout pregnancy.
Cord blood samples were taken at delivery.

The researchers observed a significant inverse relationship between total T3 blood levels in pregnant women and low environmental exposure to various pollutants including certain PCBs, pp-DDE, hexachlorobenzene, and inorganic mercury.

They found no significant relationship between these organic pollutants and cord blood T3, but the authors suggest that this lack of correlation “ could be related to other biological factors not measured in the present study. ”

“ Our findings place emphasis on the need to be more vigilant in regard to particularly fragile populations such as pregnant women and fetuses, ” said study coauthor Larissa Takser, of Laboratoire de Physiologie Materno-foetale, Montreal.
The authors conclude that the potential of low-dose exposure to organochlorine mixtures to interfere with hormonal status during pregnancy warrants further investigations with complete assessment of thyroid status to determine the short- and long-term consequences of these disturbances.

The research has been published in EHP ( Environmental Health Perspectives ) by the National Institute of Environmental Health Sciences, part of the U.S. Department of Health and Human Services.

Source: EHP, 2005


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