How efficiently metals pass from a mother’s bloodstream through the placenta to the fetus is a better indicator of thyroid hormone disruption in newborns than the amount of metals present in the mother’s blood.

When more of certain metals and minerals pass from mother to fetus through the placenta, newborns may have higher levels of thyroid-stimulating hormone and lower levels of free thyroxine, which are indicators of altered thyroid hormone regulation.

Higher levels of thallium and arsenic in mothers, higher copper levels in the umbilical cord, and less efficient transfer of copper from mother to fetus are linked to lower levels of free thyroxine in newborns, which may indicate these factors play a key role in disrupting fetal thyroid function.

When pregnant individuals have low vitamin D levels, the harmful effects of mercury and iron crossing the placenta on the newborn's thyroid function may be stronger. Vitamin D levels might influence how the fetal thyroid responds to these metals.

Higher levels of multiple metals in a mother’s blood and her baby’s umbilical cord blood are linked to lower levels of free thyroxine in newborns, which may indicate that exposure to metal mixtures affects fetal thyroid hormone levels more than exposure to single metals.