U of Michigan Study: BPA, Phthalates Suppress Thyroid

Exposure to phthalates and bisphenol-A (BPA) —two common chemicals found in plastic—can reduce thyroid hormone levels by as much as 10%, according to researchers at the University of Michigan School of Public Health.

Drs. John Meeker and Kelly Ferguson looked at serum thyroid hormone levels, and biomarkers of phthalate and BPA exposure in the urine of 1,346 adults over age 20, and 329 teenagers. They found significant inverse relationships between metabolites of these plasticizing agents and total T4, free T4, T3, and thyroglobulin. There was a clear linear relationship between the plastic metabolites and thyroid-stimulating hormone.

Phthalates and BPA are commonly used to make plastics used in food containers, beverage bottles, and linings for a wide variety of cans and containers. These agents are known to have estrogen-mimicking effects, and previous researchers have suggested that they may affect thyroid function as well.

The Michigan study, which drew its subjects from the NHANES registry, is the largest study of this issue to date, and the data are unequivocal. The largest and most consistent change was in total thyroxine (T4) levels: adults and adolescents inthe highest quintile for exposure to phthalates showed T4 levels that were on average 10% lower than the subjects in the lowest quintile for phthalate exposure. This inverse relationship was more consistent among adults than adolescents (Meeker J, Ferguson K. Environ Health Perspect. 2011; 119(10): 1396-1402).

Dr.Meeker believes the findings have public health significance, given the consistency of the observed thyroid dysregulation, and the wide use of plastics containing phthalates and BPA in the food and beverage industries. These environmental exposures could be a contributor to the rising tide of thyroid problems in this country.

However, he stops short of claiming a direct cause and effect relationship based on the current study because the urine and serum samples were collected at a single point in time. One snapshot, so to speak, cannot definitively prove a causal relationship, but it is highly suggestive.

This subject clearly warrants further attention from researchers, clinicians and public health policy-makers.