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Perchlorate Impacts Thyroid at Low Doses

A common ingredient in rocket fuel, perchlorate has contaminated groundwater across the United States. It's been found in milk, fruit, and vegetables, as well. At high levels, perchlorate can inhibit the function of the thyroid gland, leading to hypothyroidism in adults. Now, a large study published yesterday in Environmental Health Perspectives shows that much lower levels of perchlorate, traditionally considered safe, can be detrimental to thyroid function in women. "It's stunning," says endocrinologist Thomas Zoeller of the University of Massachusetts, Amherst.

Perchlorate interferes with the thyroid gland's uptake of iodine, which it uses to make the hormone thyroxine that regulates metabolism. The effects in the new study were seen in women who consume less than 100 micrograms of iodine a day, an amount below which, on a population scale, is associated with hypothyroidism, according to the World Health Organization. Thirty-six percent of women in the United States fall below that figure. While overall thyroid function was still in normal ranges in these women, the changes are of concern for pregnant women. Fetuses are particularly sensitive to changes in thyroxine, which is important for neural development.

The study was conducted by the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. As part of a regular health survey, researchers measure the levels of some 300 chemicals in a representative sample of people. The new study, led by CDC's Benjamin Blount and Jim Pirkle, examined the urinary levels of perchlorate and blood levels of thyroxine and thyroid stimulating hormone (TSH) in 2299 men and women aged 12 and older.

Perchlorate had no apparent impact on thyroid hormones in men. For women, higher levels of perchlorate correlated with more TSH. Because iodine deficiency heightens the effect of perchlorate, the team more closely examined those women with less than 100 micrograms iodine per liter of urine. The thinking went that these women might be especially sensitive to perchlorate. For these women, perchlorate was strongly correlated with a small-to-moderate fall in thyroxine and a similar-sized rise in TSH. (When thyroxine is falling, TSH stimulates the thyroid to make more.) "We were not expecting to see the effect," Pirkle says, because previous studies--which tended to combine men and women in the analysis--had shown no effects at higher levels of perchlorate. It's not known why women are more susceptible, but they have higher rates of hypothyroidism than men do.

CDC is planning a follow-up study to confirm the findings; it is also planning to investigate levels in pregnant women and cord blood of neonates. Meanwhile, Pirkle notes that just a half teaspoon of iodized salt provides an adequate amount of iodine to minimize perchlorate's hazardous effects.

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