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 T(4) (FT(4)), and thyroperoxidase antibody were measured.
Results: Urinary iodine was low: median 98 microg/liter in Cardiff and 52 microg/liter in Turin. Urine perchlorate was detectable in all women. The median (range) urinary perchlorate concentration was 5 microg/liter (0.04-168 microg/liter) in Turin and 2 microg/liter (0.02-368 microg/liter) in Cardiff. There were no associations between urine perchlorate concentrations and serum TSH or FT(4) in the individual euthyroid or hypothyroid/hypothyroxinemic cohorts. In multivariable linear analyses, log perchlorate was not a predictor of serum FT(4) or TSH.
Conclusions: Low-level perchlorate exposure is ubiquitous but did not affect thyroid function in this cohort of iodine-deficient pregnant women.