Perchlorate and thiocyanate exposure and thyroid function in first-trimester pregnant women

Context: Thyroid hormone, requiring adequate maternal iodine intake, is critical for fetal neurodevelopment. Perchlorate decreases thyroidal iodine uptake by competitively inhibiting the sodium/iodide symporter. It is unclear whether environmental perchlorate exposure adversely affects thyroid function in pregnant women. Thiocyanate, derived from foods and cigarette smoke, is a less potent competitive sodium/iodide symporter inhibitor than perchlorate. Objective: Our objective was to determine whether environmental perchlorate and/or thiocyanate exposure is associated with alterations in thyroid function in pregnancy. Design and Setting: We conducted a cross-sectional study at health centers in Cardiff, Wales, and Turin, Italy. Patients: During 2002–2006, 22,000 women at less than 16 wk gestation were enrolled in the Controlled Antenatal Thyroid Screening Study. Subsets of 261 hypothyroid/hypothyroxinemic and 526 euthyroid women from Turin and 374 hypothyroid/hypothyroxinemic and 480 euthyroid women from Cardiff were selected based on availability of stored urine samples and thyroid function data. Main Outcome Measures: Urinary iodine, thiocyanate, and perchlorate and serum TSH, free T4 (FT4), and thyroperoxidase antibody were measured. Results: Urinary iodine was low: median 98 μg/liter in Cardiff and 52 μg/liter in Turin. Urine perchlorate was detectable in all women. The median (range) urinary perchlorate concentration was 5 μg/liter (0.04–168 μg/liter) in Turin and 2 μg/liter (0.02–368 μg/liter) in Cardiff. There were no associations between urine perchlorate concentrations and serum TSH or FT4 in the individual euthyroid or hypothyroid/hypothyroxinemic cohorts. In multivariable linear analyses, log perchlorate was not a predictor of serum FT4 or TSH. Conclusions: Low-level perchlorate exposure is ubiquitous but did not affect thyroid function in this cohort of iodine-deficient pregnant women.