Low-level perchlorate exposure not linked to alterations in thyroid function during pregnancy

Exposure to low-level perchlorate was ubiquitous in pregnant women in a recent study, but was not associated with alterations in thyroid function for iodine-deficient women in the first trimester.

“The Environmental Protection Agency and various state agencies are currently considering whether the perchlorate content of drinking water should be regulated. Regulatory determinations to date have been based on relatively limited human data,” the researchers wrote.

To determine whether exposure to environmental perchlorate and thiocyanate is linked to thyroid difficulties during pregnancy, researchers enrolled 22,000 women at less than 16 weeks gestation during 2002 to 2006 in the Controlled Antenatal Thyroid Screening Study. The study also included subsets of 374 hypothyroid/hypothroxinemic women and 480 euthyroid women at health centers in Cardiff, Wale, and 261 hypothyroid/hypothyroxinemic women and 526 euthyroid women at centers in Turin, Italy.

The researchers measured urinary iodine, thiocyanate, perchlorate, serum thyroid stimulating hormone, free thyroxine and thyroperoxidase antibodies.

Median urinary iodine values were low in all women — 98 mcg/L in the Cardiff population and 52 mcg/L in the Turin population.

Urine perchlorate was detected in all women, with a median concentration 5 mcg/L in the Turin population and 2 mcg/L in the Cardiff population.

Associations between urine perchlorate concentrations and serum TSH or free T₄ were not reported in the individual euthyroid or hypothyroid/hypothyroxinemic cohorts. Following adjustment, log perchlorate was not a predictor of serum free T₄ or TSH.

Interpreting the results

Gregory Brent, MD, professor of medicine and physiology at UCLA School of Medicine, discussed concerns related to perchlorate exposure, which can be found in municipal water supplies, crop irrigation and a number of manufactured products, in an accompanying editorial. He said these data provide reassurance that low-level perchlorate exposure does not alter thyroid function at a susceptible time of pregnancy.

However, there may be further cause for concern because the effects of perchlorate in pregnancy unrelated to maternal thyroid function may be at work impacting the thyroid hormone regulation of fetal brain development, according to Brent.

“Perchlorate crosses the placenta and may influence fetal thyroid function, acting directly on the fetal thyroid gland or by influencing iodide transport. Fetal thyroid reserve and capacity for compensation are less than those of the adult,” he wrote.

He recommended perchlorate exposure as a variable in studies that examine child development and subsequent investigation to further define the apparent differences in the influence of perchlorate in studies from Europe and the United States.

“As these studies are being addressed, women of reproductive age would be best served by ensuring adequate iodine intake, usually requiring a vitamin containing iodine,” he wrote.

Pearce EN. J Clin Endocrinol Metab. 2010:95;3207-3215.

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