Maternal essential metals, thyroid hormones, and fetal growth: Association and mediation analyses in Chinese pregnant women

Background Essential metals play critical roles in fetal growth and development, but results from human studies are inconsistent. Additionally, whether maternal thyroid hormone (TH) levels mediate the associations between essential metals and fetal growth remains unknown. Methods Data for analysis were extracted from the Information System of Guangdong Women and Children Hospital between January 2017 and December 2019. Maternal levels of essential metals [copper (Cu), zinc (Zn), magnesium (Mg), and iron (Fe)] and THs were measured at the second trimester. Multivariate linear models were introduced to evaluate the potential associations between maternal essential metals, thyroid functions, and fetal growth, and the possible mediation effects of thyroid functions were explored in the median analyses. Results A total of 4186 mother-infant pairs were included in the present study. Maternal Fe levels were found to significantly increase birth weight in 272.91 g (95 % CI: 15.59, 530.22) among anemia group. Maternal Cu levels were positively associated with increased free triiodothyronine/free thyroxine ratio (FT3/FT4). Negative associations of Fe and Mg levels with thyroid-stimulating hormone (TSH) concentrations were observed, accompanied with the positive associations in relation to FT3, FT4 and FT3/FT4 ratio. Mediation analyses suggested that 72.01 % of the associations between Fe levels and birth length might be mediated by FT3 levels. Additionally, 25.85 % of the Cu-birth length association and 44.53 % of the Fe-birth length association could be explained by FT3/FT4 ratio. Conclusion Our findings suggest that maternal Cu, Mg, and Fe levels can alter TH concentrations, and maternal FT3 and FT3/FT4 ratio might be potential mediators on the developmental effects of Cu and Fe levels.