A new study shows that early-life exposure to certain phthalates — a group of chemicals found in a wide variety of household items including shampoos, perfumes, nail polish, plastic toys, house building materials and more — is linked to lowered thyroid function in young girls.

The new study, published in the journal Environment International looked at 229 women during pregnancy and 229 children who were three years old. The researchers found that exposure to a common group of phthalates in childhood was associated with lower levels of active thyroid function in 3-year-old girls. In general, phthalates are thought to be endocrine disruptors, which means they interfere with the body’s hormones. Thyroid function is important because thyroid hormones help control proper brain development.

“I think the message to consumers is be careful of the products they use,” says study author Pam Factor-Litvak, a professor of epidemiology at Columbia University’s Mailman School of Public Health. “Depressed thyroid hormones are associated in many studies with feelings of depression, anxiety and behavior problems in children as well as metabolic issues later in life.”

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The researchers did not find the same connection between phthalates exposure and lower thyroid function among boys, or the same effects from prenatal exposures. Factor-Litvak says that thyroid-related problems are more common among women than men, and that girls may be more susceptible to chemicals’ effects on thyroid function, though that is still speculative. In past studies, researchers at the Columbia University have linked phthalates exposure to lower IQ at a young age, asthma during childhood and mental development issues.

Avoiding phthalates exposure can be tough because it’s not currently required that they be listed on product labels, but products that list “fragrance” tend to contain them. While many companies are replacing phthalates in their products, Factor-Litvak says there’s still concern about what the replacement ingredients might be, and whether they are safer. To better understand how exposure could impact thyroid function, Factor-Litvak and her colleagues plan to continue to study the children as they grow older.

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