A link between chemicals called phthalates and thyroid hormone levels was confirmed by the University of Michigan in the first large-scale and nationally representative study of phthalates and BPA in relation to thyroid function in humans.

The U-M School of Public Health study also reported suggestive findings consistent with a previously reported link between a chemical called bisphenol-A and thyroid hormone levels. BPA is best known for its use in certain plastic water bottles and in the linings of canned foods.

Researchers used publicly available data from the U.S. National Health and Nutrition Examination Survey to compare urine metabolites and serum thyroid measures from 1,346 adults and 329 adolescents. Generally speaking, greater concentrations of urinary phthalate metabolites and BPA were associated with greater impacts on serum thyroid measures, said John Meeker, assistant professor at U-M SPH and lead study author.

Specifically, researchers found an inverse relationship between urinary markers of exposure and thyroid hormone levels, meaning as urinary metabolite concentrations increased, serum levels of certain thyroid hormone levels decreased.

Phthalates and BPA are chemical compounds that appear in solvents, plasticizers and common household products. These latest results were consistent with findings from previous smaller studies by Meeker and others that suggested the relationship.

The current study showed the strongest relationship between thyroid disruption and DEHP, a phthalate commonly used as a plasticizer. Research has shown that the primary exposure to DEHP is through diet. Urine samples in the highest 20 percent of exposure to DEHP were associated with as much as a 10 percent decrease in certain thyroid hormones compared to urine samples at the lowest 20 percent of exposure.

"This seems like a subtle difference," Meeker said, "but if you think about the entire population being exposed at this level you'd see many more thyroid related effects in people."

Researchers looked at another phthalate called DBP but overall, didn't find a significant relationship between exposure and thyroid measures. DBP is also a plasticizer, and is also used in solvents and personal care products.

Thyroid hormones play an important role in many body functions, from reproduction to metabolism and energy balance.

While the study focused primarily on adults, these findings underscore the need for more research on adults, pregnant women, and children, Meeker said, because fetal and child development may be particularly vulnerable to disruptions in thyroid hormone levels associated with exposure to environmental chemicals.

Meeker pointed out that the study had limitations. Since urine and serum samples were collected at a single point in time, researchers couldn't conclude a cause-and-effect relationship; it would be better to follow people over time and collect several samples, especially since these chemicals metabolize quickly and one snapshot may not represent the true chemical exposure.

The group has several ongoing studies on the potential impacts of phthalate and BPA exposure on pregnancy outcomes and child development.

Phthalate studies point to several health problems

Researchers from Brigham and Women's Hospital published a report in Environmental Health Perspectives which revealed that women with high phthalate levels in their urine have a considerably higher risk of developing diabetes, compared to those with the lowest levels. (Link to article)

Researchers from the Children's Environmental Health Center at The Mount Sinai Medical Center found a link between obesity in young children and exposure to phthalates. (Link to article)

Scientists from the University of Rochester Medical Center found in a pilot study that young boys whose mothers had high phthalate urine levels when they were pregnant were less likely to "play like boys" - engage in boyish banter, play fighting, and use masculine toys. (Link to article)

Phthalate concentrations were found in infants' urine by researchers from the CDC in Atlanta and the University of Rochester School of Medicine and Dentistry. The investigators said phthalates are widely used in baby care products, such as lotion, talc and shampoo. (Link to article)

A Finnish study found a link between phthalates and diabetes risk among elderly people; even when circulating phthalate levels were only moderately elevated their risk doubled. (Link to article)

At Columbia University's Mailman School of Public Health, scientists found "prenatal exposure to phthalates may adversely affect child mental, motor and behavioral development during the preschool years". (Link to article)

Scientists from the University of Michigan, in a large-scale study, confirmed a link between phthalate and BPA concentrations and thyroid hormone levels. Thyroid hormones play a key role in reproduction, metabolism, energy balance and other body functions. (Link to article)