In-Utero Exposure to High Magnetic Fields and the Risk of Abnormal Thyroid Hormone Levels in Offspring: A Prospective Cohort Study with up to 20 Years of Follow-Up

Abstract

Background/Aim: Exposure to magnetic fields (MF) and hypothyroidism in children have been increasing in parallel. Animal studies have shown that MF exposure could impact thyroid hormone levels. Methods: The impact of maternal MF exposure during pregnancy on thyroid hormones in offspring was examined in a prospective cohort study among members of the Kaiser Permanente Northern California (KPNC) healthcare system. Pregnant women were asked to carry an EMDEX meter for 24 hours during early pregnancy to capture MF exposure from all sources. Confounders were ascertained during in-person interviews. Among 1,063 participants, 734 had both a live-born child and valid MF measurements, thus were included in the current study. During 20 years of follow-up, 96 children had laboratory test results for TSH and free T4 thyroid hormones. Those whose TSH and T4 levels were both below the median level were classified as having lower thyroid function. Results: After controlling for multiple confounders using Cox survival analysis using all 734 participating children, those with high in-utero MF exposure [95th percentile MF ≥ 2 milligauss (mG)] had an increased risk of having low thyroid hormone levels [hazard ratio (HR)=2.7, 95% confidence interval (CI): 0.9-7.8] compared to children with lower in-utero MF exposure (< 2 mG). When restricting the analysis to the children who had both TSH and T4 test results, the association was stronger and statistically significant (HR=3.1, 95%CI: 1.1-9.0). There was also a dose-response relationship: HR=2.3 for in-utero MF level 2-4 mG, and HR=5.0 for in-utero MF level > 4 mG). Conclusions: In-Utero exposure to high MF level is associated with increased risk of low thyroid hormone levels, mostly due to low TSH levels. These results indicate a possible adverse impact of MF on the pituitary gland during in-utero fetal brain development. This important novel finding needs be further examined in future studies.