Dioxins meddle with key thyroid hormone.

Dioxins appear to tamper with infants' thyroid systems even in low concentrations, spelling potential trouble for babies' psychomotor development, researchers in the Netherlands report.

"It's the first time anyone has really picked up on thyroid status in relation to dioxin concentrations" in humans, comments James D. McKinney of the Environmental Protection Agency in Research Triangle Park, N.C. Other studies have shown that dioxins affect thyroid hormone concentrations in animals.

The new study, published in the November ENVIRONMENTAL HEALTH PERSPECTIVES, finds that babies exposed to greater amounts of dioxins have higher, although still normal, concentrations of a key thyroid hormone in their blood. These infants appeared healthy at 6 months, says Hendrik J. Pluim of the Academic Hospital of the University of Amsterdam, one of the study's authors. But the effect concerns Pluim and his colleagues because of the importance of thyroid hormones to development, he says, so they continue to monitor the infants.

McKinney says that the researchers may not have picked up all of the dioxins' effects. The study looks only at thyroid function, whereas "the real issue is what is happening at the level of the cells," he says. Scientists had already found that dioxins can affect the body's immune system and other hormones (SN: 1/11/92, p.24).

In the new study, the researchers measured the concentrations of seven dioxins and 10 dibenzofurans, which have chemical properties similar to those of dioxins, in the breast milk of 38 mothers of newborns. They classified the infants as being in either a high- or low-exposure group, depending on the concentrations of these chemicals in their mothers' milk. Dioxin concentrations in all of the milk fell within the normal range for the population, Pluim says.

The researchers then measured thyroid hormone concentrations and other indicators of thyroid function in the infants' blood at birth and at 1 week and 11 weeks of age.

The high-exposure group showed greater concentrations of [T.sub.4], the hormone most synthesized by the thyroid gland, one week and 11 weeks after birth. At 11 weeks, the babies also had higher concentrations of thyroid-stimulating hormone (TSH), which causes the thyroid gland to produce [T.sub.4], the researchers write. Furthermore, the ratio of [T.sub.4] to a protein that transports [T.sub.4] was higher, the group notes.

"We postulate that dioxins influence thyroid hormone concentrations in infants by interfering with the thyroid hormone regulatory system," they write. For example, children in the high-exposure group had both higher [T.sub.4] and TSH, even though [T.sub.4] normally inhibits the release of TSH, Pluim says.

They hypothesize that dioxin encourages the release of [T.sub.4] in the pituitary gland. This and other disruptions of the thyroid regulatory system by dioxin could threaten normal psychomotor development and the maturation of the central nervous system, Pluim warns.