Studies on the effect of pentobarbiturate, ether and heparin on plasma thyroxine level in rats

In groups of 9 to 20 adult male rats decapitated at 0, 15 or 60 min after i.p. injection of saline or at the same intervals after 2 min exposure to ether no differences in plasma thyroxine level were found. In groups of 5-6 animals a significant decrease of thyroxine level in plasma was observed at 30 min after i.p. injection of pentobarbiturate (40 mg kg-1) and unsignificant decrease after 800 and 2400 U heparin i.p. per animal. Finally, in 58 animals a linear decrease of plasma thyroxine level was found during 480 min beginning from about 60th to 90th min after the onset of pentobarbiturate anaesthesia (40 mg kg-1 followed by 20 mg kg-1 every 60-90 min). It was concluded that the blood clotting preventive dose of heparin (i.e. 300 U per 400 g animal) presumably does not interfere with the level of thyroxine in plasma, while pentobarbiturate apparently results in a considerable decrease of that.