Oct. 5, 2006 -- Environmental exposure to a common pollutant found in milk and drinking water appears to directly affect hormone levels in women.
Researchers of a CDC study say they were surprised by their findings that the chemical, called perchlorate, consistently hinders thyroid function after women consume everyday doses in food and water.
Perchlorate, used in rocket fuel, is widely known to damage the thyroid of animals at high doses in laboratory studies. But scientists, environmental groups, and the defense industry have long been at odds over whether it poses a health risk at relatively low levels found in the environment.
"We thought the low levels would lead to very low or trivial effects and that happened not to be the case," James L. Pirkle, PhD, tells WebMD. Pirkle is the deputy director for science at the CDC's Environmental Health Laboratory.
Perchlorate can alter hormone levels by partially blocking absorption of iodide in the thyroid. Iodide is a chemical that's vital for the production of thyroid hormone.
Blocking iodide absorption can lead to hypothyroidismhypothyroidism -- an underactive thyroid -- and goiter in adults. A dysfunctional thyroid during pregnancypregnancy can lead to abnormal brain development and preterm birth in children.
In the study, researchers found that rising levels of perchlorate in the urine of 1,111 women were directly linked to rising levels of thyroid stimulating hormone (TSH), an indication of an underactive thyroid.
The effect was even stronger in women with iodide deficiency, which affects roughly one-third of the population. In those women, perchlorate exposure was linked to consistent decreases in the thyroid hormone known as thyroxine -- again, consistent with hypothyroidism.
The study found no similar result in men.
Pirkle says the findings suggest that everyday perchlorate exposure in the general population has "a small to medium effect" on women's thyroid functioning. He said the surprising results were fueled mostly by the use of a new, highly sensitive perchlorate test.
"We really felt like at these low levels we were not going to see an effect," he says.
Iodide's Protective Effects
The study's findings suggest that women can protect themselves against the effects of perchlorate exposure by "just maintaining an adequate level of iodide in the diet," Pirkle says. About a half-teaspoon of iodized salt per day is considered enough to raise iodide to normal levels in most adults.
Gregory Brent, MD, who chaired a 2005 National Academy of Sciences panel on potential health effects of perchlorate, calls Thursday's study "important."
Based on the study, Brent says, "it's fair" to conclude that environmental levels of perchlorate have the potential to affect human health.
But since the results did not extend to men, "there should be a caution about how generalizable the results are," says Brent, a professor of physiology at the University of Southern California.
Perchlorate is found in solid rocket propellant but also occurs naturally. In 2005, the Environmental Protection Agency (EPA) set a safe limit for perchlorate in drinking water at 24.5 parts per billion (ppb), though regulators have not yet enacted drinking water standards for the chemical.
This summer, Massachusetts set a drinking water standard at 2 ppb. EPA spokesman Dale Kemery says the agency is looking at the possibility of additional study of perchlorate but that there are no plans to revise the existing perchlorate limit.
He says the "EPA is interested in the CDC's findings, although CDC scientists recommend that their study be confirmed with additional research."
The Environmental Working Group, a regulatory watchdog organization, says in a statement that 44 million women with low iodide or who are pregnant are at potential risk of perchlorate health effects.
"This new study shows that even very small levels of perchlorate in water or food can have a marked effect on thyroid levels in women. We can't ignore this serious public health issue any longer," says Renee Sharp, an analyst with the group.
Industry groups have long maintained that perchlorate pollution in the environment poses no risk to humans. The National Defense Industrial Association did not respond to requests for a comment.
"One of the unknown issues is how much perchlorate is out there in the environment," Brent says.