Altered Thyroid Levels May Affect Fetal Brain Development

A recent study by biologist R. Thomas Zoeller of the University of Massachusetts Amherst reports compelling evidence that endocrine disrupting chemicals such as polychlorinated biphenyls (PCB) found in flame retardant cloth, paint, adhesives and electrical transformers can interfere with thyroid hormone action in pregnant women and may travel across the placenta to affect the fetus.

Zoeller states that “as endocrine-disrupting chemicals, PCBs interfere with the way the thyroid hormone functions, but they don’t actually change the amount of the hormone found in the body. Although these effects are largely invisible in scientific studies that only judge thyroid activity by measuring hormone levels, they may be having a real impact on infants’ brain development.”

Endocrine-disrupting PCBs were banned in the United States in 1979, however they are still released into the environment from disposal sites or products made before the ban. Most people have been exposed to low levels of PCBs, Zoeller points out.

In the study, the effects of low-dose chemical exposure in 164 pregnant women were observed. Tissue from their placentas, the uterine structure that provides oxygen and nutrients to the fetus was analyzed for the enzyme, CYP1A1. This enzyme changes endocrine-disrupting chemicals into a form that can interfere with the body’s thyroid hormone receptors.

Zoeller and colleagues discovered that in pregnancies where the placenta contained higher levels of CYP1A1, it also showed signs of thyroid disruption. Levels of two thyroid-regulated genes tended to be higher in these pregnancies, although the mother’s overall thyroid hormone levels did not change.

“Whatever is happening in the placenta likely reflects what is happening in the fetus,” says Zoeller. “To truly understand how endocrine-disrupting chemicals may be affecting pregnancies, the findings show we need to study not only hormone levels, but hormone activity at the cellular level.”

The effects of endocrine-disrupting chemicals could be especially dangerous in people who smoke. The enzyme CYP1A1 is supposed to clean the blood, and the body produces more of this enzyme when it is exposed to cigarette smoke. The researchers found pregnant women who smoked tended to have higher levels of the enzyme in the placental tissue.

Source: University of Massachusetts at Amherst