Industrial Chemicals Alter Placental Thyroid Hormone Activity

Marlene Busko

October 09, 2014

Certain industrial chemicals commonly found in low levels in the environment can infiltrate the placenta during pregnancy and affect thyroid hormone activity in placental tissue, a new study reports.

And since thyroid hormones play a key role in fetal brain development, maternal exposure to these endocrine-disrupting chemicals might affect cognitive development in infants, the researchers speculate. However, they caution that this remains to be proven.

Specifically, the researchers measured levels of the CYP1A1 enzyme in placental tissue. This enzyme transforms endocrine-disrupting chemicals such as polychlorinated biphenyls (PCBs) and other dioxinlike industrial chemicals into a form that can interfere with thyroid hormone receptors.

"It is clear that thyroid hormone action is being derailed by environmental factors," senior author R. Thomas Zoeller, PhD, of the University of Massachusetts in Amherst, told Medscape Medical News. "We know that CYP1A1 is influenced by [dioxinlike] environmental chemicals."

The next step in their ongoing research will be to see whether poorer cognitive development in toddlers correlates with higher CYP1A1 levels in placental tissue.

The researchers also found that pregnant women who smoked had higher levels of CYP1A1 in placental tissue, thus providing yet another reason to quit smoking.

"If [a pregnant woman] is also a smoker or if [she is] exposed to higher levels of dioxinlike compounds, then [her] system is going to activate these ubiquitous chemicals, and [this will] interfere with thyroid hormone action," Dr. Zoeller said.

The study was published online October 9 in the Journal of Clinical Endocrinology and Metabolism.

Environmental Toxins and Thyroid Hormone Activity in Pregnancy

The researchers analyzed placental samples from 164 pregnant women who had no thyroid disease and who participated in the GESTE Canadian prospective birth cohort study. The women had a mean age of 28 years.

Maternal and cord-blood thyroid hormone levels were measured at birth, and the messenger RNA (mRNA) levels of CYP1A1 and placental thyroid-hormone–receptor targets were measured.

Of these 164 placental samples, CYP1A1 was detected in 132 samples (80%), and levels of 2 thyroid-hormone–regulated genes tended to be higher in these pregnancies, although the mother's overall thyroid hormone levels did not change, nor did the levels of thyroid hormone in cord blood.

The implication is that PCBs and other endocrine-disrupting chemicals can infiltrate the placenta during pregnancy and affect thyroid hormone activity at the cellular level.

"The present data support the concept that environmental chemicals may interfere with thyroid hormone action by a mechanism that is not limited to or revealed by changes in serum thyroid hormone levels," Dr. Zoeller and colleagues write.

The study also hints at how prenatal maternal exposure to environmental chemicals such as PCBs might negatively affect brain development in the offspring.

"If a similar chain of events occurs "in other tissues such as the fetal brain, it could explain the relationship between prenatal exposure to PCBs and cognitive deficits in the offspring," they conclude.

The authors have reported no relevant financial relationships.

J Clin Endocrinol Metab. Published online October 9, 2014. Available here.