[The activity of thyroxine conversion to triiodothyronine in the liver and kidneys of rats].

Rat liver and kidney microsomal fractions contain 5'-deiodinase, converting thyroxin into 3,5,3'-triiodothyronine. In control animals a marked decrease in thyroxin concentration takes place when the blood passes through the liver and kidney (triiodothyronine level being preserved). Fasting causes a prominent decrease in blood thyroxin, a small decrease in triiodothyronine as well as a drop of glucose and insulin levels. At the same time a decrease in thyroxin deiodinase activity takes place in the liver; as for the kidney a decrease is less marked. Fasting rats have no gradients in the blood passing through the liver and kidney; triiodothyronine balance remains unchanged. The results show that the rat liver and kidney deiodinase activity regulation (during changes in the body physiological status) helps to maintain triiodothyronine optimum concentration in general circulation.