Immediate Opposite Effect of Salicylate on Thyroxine and 3,5,3′-Triiodothyronine versus 3,3′,5′-Triiodothyronine Level in Plasma in Rats

Abstract

In a total of 46 male rats polyethylene tubings were introduced into femoral artery and vein under pentobarbiturate anesthesia. Then heparin (300 U kg-1) was injected at 60-90 min after pentobarbiturate and two control blood samples were subsequently taken. After that sodium salicylate (200 mg kg-1) was injected i.v. and blood samples were taken at 30-420 min later. An immediate decrease of the thyroxine (T4) level in plasma to about 20% of original level and that of 3,5,3'-triiodothyronine (T3) to about 60% of that was found, while the level of 3,3',5'-triiodothyronine (rT3) was increased 20%. It was concluded that the administration of salicylate results in an immediate displacement of T4 and T3 from plasma protein binding and possibly inhibits the conversion of T4 to T3 and of rT3 to 3,3'-diiodothyronine which results in an increase of rT3 level in plasma. This might by partially prevented by an inhibiting effect of salicylate on the binding of rT3 to plasma proteins.

In Vivo Study of Iodothyronine Deiodination in Rat Liver: Effect of Salicylate on Biliary Excretion of Several Iodothyronines

“…An immediate increase of T 4 excretion by bile after salicylate injection may be due to an acute displacement of this substance from plasma as described previously (Langer et al 1976(Langer et al , 1981 and to an increased excretion of bile found in these experiments which presumably results from increased blood flow through the liver. In the case of T 3 the increase of its biliary excretion during the first 2 h period after salicylate administration was much less remarkable than that found in the case of T 4 .…”