Several common environmental chemicals disrupt thyroid function in pregnant women and affect brain development in the fetus, scientists suggested at a recent press briefing. Some learning disabilities in children may result from an untreated thyroid disorder experienced by their mothers during pregnancy. Chemicals such as polychlorinated biphenyls and perchlorate, as well as the flame retardants polychlorinated biphenyl ethers and tetrabrominated bisphenyl A, bind to the thyroid receptor and disrupt thyroid function, the scientists said. "We are concerned about subclinical hypothyroidism--a condition in which thyroid-stimulating hormone is elevated--in pregnant women because thyroid hormone is essential for normal fetal brain development," said Ted Schettler, physician and science director for the Science & Environmental Health Network. According to James Haddow, clinical professor of pediatrics at the University of Vermont School of Medicine, 2.5% of mothers have subclinical hypothyroidism. Their offspring have IQs that are seven to eight points lower than they would otherwise be, he observed. The scientists also urged that the National Children's Study be fully funded and completed because it will cast much more light on how environmental chemicals affect thyroid function. So far, Congress has not appropriated funds to move the study beyond the planning stage (C&EN, April 26, page 21).