Prenatal levels of 6 PCBs were positively correlated with cord serum T3.
4 PCBs showed a negative correlation with cord serum reverse T3.
5 PCBs were positively correlated with cord serum T3/rT3 ratio.
Prenatal exposure to PCBs might reduce deiodinase 3 activity.
Reduction of deiodinase 3 activity is a potential mechanism of PCB toxicity in humans.