Interactive effects of 2,2′,4,4′,5,5′-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin on thyroid hormone, vitamin A, and vitamin K metabolism in the rat

Groups of rats were fed diets containing various concentrations of 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD) or 2,2,′,4,4′,5,5′-hexachlorobiphenyl (PCB 153) alone or in combination, for 13 weeks, with the aim to investigate possible interactive effects on hormonal and vitamin plasma and liver levels. Decreases were found in total (TT4) and free (FT4) thyroxin plasma levels for both compounds, which were additive after four weeks. After 13 weeks however, the effect on TT4 and FT4 was synergistic. In addition, an increase in plasma retinol, a decrease in liver retinol and a sharp decline in liver retinylpalmitate concentrations were found after TCDD or PCB 153 administration. Coadministration resulted in an antagonistic effect of TCDD on the decrease in liver retinylpalmitate levels due to PCB 153.