Competitive binding to plasma thyroid hormone transport proteins and thyroid disruption by phenylbutazone used as a...

A model of thyroidectomized sheep intravenously supplemented with thyroid hormone (TH) was developed to mimic endogenous TH exposure and to analyze the impact on plasma TH homeostasis of xenobiotic interference with TH binding to plasma proteins. TH was displaced from plasma protein binding sites by using phenylbutazone (PBZ) as a test xenobiotic, to compare the effect of PBZ on steady state free and total plasma TH concentrations between the in vivo situation and an in vitro system. While PBZ increased free TH in vitro, PBZ administration in vivo produced an immediate reduction in both total and free plasma TH. The decrease in the total TH was consistent with a PBZ-induced displacement of TH from its plasma binding proteins, leading to an increase in total TH plasma clearance. However, this reduction in total TH was not expected to be accompanied by a parallel decrease in free plasma TH since the free TH is determined by the clearance of the free plasma TH. This suggested that PBZ may also have interfered with the clearance mechanisms of free TH. It can be concluded that our thyroidectomized sheep model enables a dual action of a xenobiotic on plasma TH to be distinguished, namely a displacement of TH from its binding proteins leading to a decrease in the total plasma concentration, which is not relevant to thyroid function versus an interference with the intrinsic TH clearance leading to a change in the free plasma TH, which has a major impact in terms of thyroid disruption.