Kinetic parameters of L-[125I]triiodothyronine degradation in rats pretreated with polyhalogenated biphenyls.

These studies were designed to elucidate the effects on thyroid function and thyroid-hormone activity of the long-term administration of low doses of four polyhalogenated aromatic compounds. The compounds studied were the polychlorinated biphenyls (PCBs) Aroclor 1254 and Aroclor 1242, and the polybrominated biphenyls (PBBs) hexabromobiphenyl and octabromobiphenyl. Groups of eight female Sprague-Dawley rats, specified pathogen free, were fed a diet containing 50 ppm of one of the polyhalogenated biphenyls or control diet for 7 months. Significant effects on serum triiodothyronine (T3) were observed in the group given Aroclor 1254. Analysis of kinetic data revealed a decrease in T3 degradation rate and an increase of biological half-life after long-term exposure to Aroclor 1254. The T3 distribution space was increased in both groups treated with PCB, suggesting that PCB intoxication may open additional fluid pools to T3, possibly because of cell damage. Analysis of values for the same parameters for rats treated with PBBs showed less marked effects. The results of this study indicate that PCBs exert a direct effect on the thyroid gland and that the rate of synthesis of T3 may be reduced, and suggest that the mechanism of thyroid-hormone synthesis may be impaired.