Effect of salicylate on the biliary excretion of thyroxine in rats.

In rats in which the plasma thyroxine is labeled with I131, injections of salicylate cause an increase in the ratio of the concentrations of radioactivity in bile and plasma (B/P ratio) and a fall in plasma radioactivity. It is suggested that both effects are due to displacement of thyroxine from the main thyroxine- binding protein of the plasma by salicylate. The greatest effect produced by salicylate on the B/P ratio is not as great as the effect of injections of exogenous thyroxine. This finding is considered in relation to the mechanism whereby the biliary excretion of thyroxine increases at high plasma thyroxine concentrations.