Introduction: Iodine is important for thyroid hormone synthesis, and iodine deficiency in pregnancy may impair foetal neurological development. Perchlorate, found in some foods and everyday chemicals (e.g. fertilisers) and thiocyanate, which is found in cigarette smoke, decrease the transport of iodine from the circulation to the thyroid cells by inhibiting the sodium-iodide symporter. Environmental exposure to these substances during pregnancy may result in reduced thyroid hormone synthesis. Therefore, we aimed to explore the impact of perchlorate and thiocyanate exposure on thyroid status in a cohort of pregnant mothers from South-West England.
Methods: Urine samples were obtained from 308 women participating in a study of breech presentation in late pregnancy. They had no known thyroid disease and a singleton pregnancy at 3638 weeks gestation. Samples were analysed for urinary concentrations of iodine (UIC), perchlorate (UPC) and thiocyante (UTC). Blood samples were taken for free T4 (FT4), thyrotropin (TSH), thyroid peroxidase antibodies (TPO-Ab). Baseline data included: age, parity, smoking status, ethnicity, BMI at booking. Variables were assessed for normality and natural log transformed where appropriate.
Results: Participants had a mean (SD) age 31(5) years, median (IQR) BMI 24.4(22.0, 28.3) kg/m2, and median (IQR) UIC 88 (55, 158) mcg/l. 42% were primiparous, 10% were smokers, and 96% were Caucasian. Log transformed UPC was negatively correlated with FT4 in the whole cohort (n=308, r=-0.12, p=0.03) and in the subgroup of women with UIC <100 mcg/l (n=174, r=−0.15, P=0.04). Regression analysis with the potential confounders, smoking, TPO-Ab status, UIC and UTC, identified UPC to be negatively associated with FT4 (P=0.04). UPC was not associated with TSH, and UTC was not associated with FT4 or TSH.
Conclusion: Environmental perchlorate exposure is negatively associated with circulating FT4 levels in third trimester pregnant women. This may have an adverse impact on neurocognitive development of the fetus.