CDC finds perchlorate–iodide connection
Women with low iodide levels may be at risk for thyroid trouble when exposed to relatively low amounts of perchlorate.
In a broad sampling of the U.S. population, researchers at the Centers for Disease Control and Prevention (CDC) have found an association between measurable levels of perchlorate—a compound best known for its use in rocket fuel—and thyroid function in women with low but normal iodide levels. The study, published online in Environmental Health Perspectives on October 5, suggests that some women, particularly those of child-bearing age, may need to increase their consumption of iodine. The data also show that perchlorate is readily detected in most people.
The sources and human health effects of perchlorate are being hotly debated in the U.S. The strong oxidizer has percolated into groundwater near military installations and has been released by fireworks and roadside flares. It occurs naturally in Chilean nitrate deposits, which were used extensively for fertilizer until the 1950s, and also results from atmospheric deposition. The chemical has surfaced in milk, lettuce, and other foods around the world.
Perchlorate at high doses blocks the uptake of iodide in the thyroid, and this interferes with the production of thyroid stimulating hormone (TSH). TSH regulates metabolism by stimulating production of the hormone T4. That hormone is produced at higher levels in pregnant women, causing them to need more iodine.
CDC researchers examined data from the 2001–2002 National Health and Nutrition Evaluation Survey. They followed 1111 women within a larger sample set of more than 2500 people throughout the U.S. (data were reported in the Journal of Exposure Science and Environmental Epidemiology on October 18. Women with urinary iodine levels (a proxy for iodide uptake) of less than 100 micrograms per liter (µg/L), who had relatively low levels of perchlorate in their urine, showed depressed T4 levels and elevated TSH. The same association was not found in men.
“This is the first time a relationship between perchlorate and thyroid function has been observed at the low levels of exposure found in the general population,” says James Pirkle, who is the deputy director for science at the CDC’s National Center for Environmental Health and a coauthor of the paper. The study is also the first to examine a large enough pool of women with lower iodine levels to get statistically valid results, he says, adding that “we expect these women to be more vulnerable” because of their lower iodine levels. The World Health Organization sets the bar for median population levels of iodine at 100 µg/L in urine; about a third of women in the U.S. fall below that level.
“This is, of course, one of the most important papers to come out in the perchlorate saga,” says Purnendu “Sandy” Dasgupta of Texas Tech University, who has been instrumental in developing analytical methods for detecting perchlorate. “The quantity and quality of data is such that it leaves very little doubt. It shows very clearly that when your iodine intake is low and perchlorate is high, in the context that the authors have defined it, that your TSH levels are elevated.” Now, he says, the debate will move to whether this extent of change in TSH levels is important in a clinical sense.
Robert Utiger of the Harvard Institute of Medicine agrees that the new CDC results suggest that women with low iodine intake exposed to perchlorate may have impaired thyroid function. However, he asks, “Is it enough to be harmful? The answer I would have to give is ‘possibly’.”
Previous studies have looked at only a handful of women who were exposed to perchlorate and had low to normal iodide levels, for example, in Chile and Israel. Elizabeth Pearce of Boston Medical Center, who has examined perchlorate in Boston women, says that other issues could confound the correlation between TSH and perchlorate. Women are more likely than men to have autoimmune hypothyroidism, but antithyroid antibodies were not measured in this research, Pearce says. She also notes that the research relied on two separate labs that used two different assays, which could lead to very subtle differences in outcomes.
Regardless of the perchlorate–iodide connection, researchers recommend sufficient iodine consumption. “We know that, worldwide, iodine deficiency is the leading factor in mental retardation,” Pearce says. “My private opinion is that all vitamins should contain iodine,” says Utiger, who was part of the National Research Council (NRC) panel that recommended iodine supplements in prenatal vitamins in a 2005 report, Health Implications of Perchlorate Ingestion. Pirkle and others emphasize that about a half teaspoon of iodized salt per day meets the daily iodine requirement for most people.
California Senators Diane Feinstein and Barbara Boxer have asked the U.S. EPA to revisit perchlorate standards in light of the new CDC research. Current EPA regulations adopted the NRC reference dose for perchlorate of 0.7 µg per kilogram body weight per day. California has proposed a 6 µg/L maximum contaminant load, and Massachusetts set its regulation at 2 parts per billion last July.
