Polychlorinated biphenyl-mediated decrease in serum thyroxine level in rodents.

Effects of Kanechlor-500 (KC500), a commercial polychlorinated biphenyl mixture, on the levels of serum thyroid hormones such as total thyroxine (T(4)) and triiodothyronine were examined in male mice, hamsters, rats, and guinea pigs. Four days after a single intraperitoneal injection of KC500, significant decreases in the levels of the serum total T(4) and free T(4) occurred in all the animals examined, whereas a significant decrease in the level of serum triiodothyronine was observed only in guinea pigs among the animals examined. In addition, no significant change in the level of serum thyroid-stimulating hormone was observed in any of the rodents examined. A significant increase in the activity of hepatic T(4)-UDP-glucuronosyltransferase after the KC500 administration occurred only in guinea pigs, whereas the increase in the amount of biliary [(125)I]T(4) glucuronide after an intravenous injection of [(125)I]T(4) to the KC500-pretreated animals occurred only in rats. On the other hand, in all the rodents examined, KC500-pretreatment promoted the clearance of [(125)I]T(4) from the serum and led to a significant increase in the steady-state distribution volumes of [(125)I]T(4). Likewise, its pretreatment raised the concentration ratio (K(p) value) of the liver to serum and the liver distribution of [(125)I]T(4) in all the rodents tested. The present findings indicate that for the first time the KC500-mediated decrease in the serum T(4) level in mice, hamsters, rats and guinea pigs occurs mainly through an increase in the accumulation level of T(4) in the liver.