A link between chemicals called phthalates and lower thyroid hormone levels was confirmed by the University of Michigan in the first large-scale and nationally representative study of this relationship. Phthalates (tha-lates) are plasticizer substances combined with plastics to increase their flexibility, transparency, etc. Also found in cosmetics and other personal care products, they are known for blocking testosterone levels.
The 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 the linings of food cans, and mimics estrogen in its effect on gene expression.
The researchers used publicly available data from the U.S. National Health and Nutrition Examination Survey (NHANES) 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 lead study author John Meeker, ScD.
Specifically, the 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 (total T4, free T4, total T3, and thyroglobulin) decreased, and levels of TSH (a marker of low thyroid) increased.
(To read the full text PDF of their article, published online Jul 11 by Environmental Health Perspectives, click HERE.)
These latest results were consistent with findings from previous smaller studies Dr. 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.
Top 20% in DEHP = Up to 10% Lower Thyroid
Research has shown that the primary exposure to DEHP is through diet. Urine samples in the highest 20% of exposure to DEHP were associated with as much as a 10% decrease in certain thyroid hormones compared to urine samples at the lowest 20% exposure.
"This seems like a subtle difference," Dr. 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, Dr. 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.
Source: University of Michigan press release, Jul 11, 2011