Associations of Breast Milk Mercury, Lead and Cadmium Concentrations with Hyperthyrotropinemia in Neonates during the First Few Days after Birth

Recent studies show that exposure to heavy metals can affect thyroid function. This study aimed to assess heavy metals in breast milk during the first few days postpartum and their relationships with hyperthyrotropinemia in neonates.

In this case-control study, 82 mother-infant pairs, consisting of 21 pairs (neonates with hyperthyrotropinemia) in the case group and 61 pairs (healthy neonates) in the control group, were assessed at 3-5 days postpartum. Thyrotropin concentrations >5 mIU/L in the heel blood sample were considered as hyperthyrotropinemia. In the case group, thyrotropin concentration was also measured in the venous blood sample for confirmation. At the same time, to assess mercury (Hg), lead (Pb), and cadmium (Cd) concentrations, a breast milk sample was collected from each mother.

The median (interquartile range, IQR) thyrotropin concentrations in the heel blood sample of healthy neonates and neonates with hyperthyrotropinemia were 0.90 (0.40-1.85) and 6.80 (5.70-8.80) mIU/L, respectively (P<0.001). This value was 5.50 (5.00-6.75) mIU/L in the venous sample of neonates with hyperthyrotropinemia. None of the heavy metals in breast milk showed a significant correlation with thyrotropin concentration in the heel blood sample. However, there was a significant correlation between the concentration of Pb in breast milk and thyrotropin level in the venous blood sample (r=0.478, P=0.029). No correlations were observed between breast milk Hg and Cd concentrations and venous thyrotropin level.

The present findings indicate that the concentration of Pb in breast milk correlates with neonates' thyroid function. However, further studies with larger sample sizes are needed to confirm our results.