For years, there have been concerns about perchlorate's effect on the thyroid. (I've been covering it for almost a decade here at the About.com Thyroid Site.) The subject is controversial, however, as government regulators, environmental groups, citizen advocates, the military, and defense contractors responsible for the contamination have gone back and forth over the actual health effects, guidelines for safe standards, and how much perchlorate is acceptable in our food and water.
In a study released in 2005, a National Academy of Sciences panel determined that perchlorate affects the thyroid's ability to absorb iodine, but that the effects would only occur with exposure to high levels of perchlorate. This new CDC study, however, shows that not only is perchlorate exposure pervasive, but for the first time, has demonstrated that even low levels of perchlorate exposure -- levels common to many Americans -- can have harmful health effects on the thyroid.
Perchlorate is a byproduct of rocket fuel production that has been found to contaminate parts of the nation's drinking water supply, as well as fruits, vegetables and grains irrigated by perchlorate-contaminated water, and milk and milk products from cows that grazed on contaminated grasses.
Perchlorate can inhibit the thyroid's ability absorb iodine from the bloodstream. Iodine is a building block of thyroid hormone. Low iodine levels, and/or the gland's inability to absorb iodine, can prevent the thyroid from producing enough thyroid hormone, resulting in an underactive thyroid -- hypothyroidism. Hypothyroidism can cause weight gain, fatigue, depression, infertility, miscarriage, and is considered a risk factor for heart disease. Babies of mothers who are hypothyroid are at increased risk of cognitive and developmental problems, or, in more severe cases, cretinism and birth defects.
The CDC study looked at 2,299 men and women, aged 12 and older, who participated in the CDC's National Health and Nutrition Examination Survey (NHANES) during 2001-2002. They evaluated the perchlorate concentrations in urine, along with the thyroid hormones thyroxine (T4) and thyroid-stimulating hormone (TSH) in the bloodstream. What the researchers found was that the presence of perchlorate was predictive of thyroid hormone levels in women -- but not men.
The researchers then focused on women in with higher-iodine levels, versus a lower-iodine group. In women with higher-iodine levels, they found a slight connection between perchlorate levels and TSH. But in the lower-iodine women, there was a strong connection between perchlorate levels, and elevated TSH and low T4 -- indicative of hypothyroidism.
Women who are pregnant are at particular risk because pregnancy already puts a strain on thyroid function, and sufficient thyroid function is necessary to maintain the pregnancy, as well as avoid cognitive or developmental problems in the baby. Women who are already slightly hypothyroid are also at greater risk, because the effects of the perchlorate may worsen their existing hypothyroidism.
According to the CDC, 36% of women in the U.S. have urinary iodine levels less than 100 µg/L, which was the lower-iodine level used for the study. Looking at those with lower iodine levels, as well as women who are pregnant, and those who are borderline hypothyroid already, the Environmental Working Group has concluded that an estimated 44 million American women are at heightened risk from perchlorate exposure.
How Much Perchlorate is a Risk?According to the CDC, the median level of perchlorate found in urine was 2.9 micrograms per liter (a microgram per liter is equal to 1 part per billion). With average urine output at about 1.5 liters per day, this translates to around 5 micrograms of perchlorate per day being ingested. Even at this low level, there were negative effects seen on the thyroid.
The federal "safe dose" level, however, is almost ten times this dose.
According to the Environmental Working Group, the CDC has found that perchlorate levels in water as low as 3 parts per billion think of one teaspoon of water in an Olympic-sized swimming pool can have an effect on women's health.