Regulation of iodothyronine deiodinase activity as studied in thyroidectomized rats infused with thyroxine or triiodothyronine

To provide new insights into the in vivo regulation of iodothyronine deiodinases in the different tissues of the rat, we have evaluated the effects on these enzymatic activities of T-4 or T-3 infusions into thyroidectomized rats. Thyroidectomized rats were infused with placebo, T-4, or T-3. Placebo-infused intact rats served as euthyroid controls. Plasma and samples of cerebral cortex, brown adipose tissue, pituitary, liver, and lung were obtained after 12-13 days of infusion. Plasma TSH, plasma and tissue T-4 and T-3, and iodothyronine deiodinase activities were determined. Type II 5'-deiodinase (DII) was increased in cortex, brown adipose tissue, and pituitary from animals infused with placebo. DII activity returned to normal only with T-4 infusion, remaining elevated in the animals infused with T-3 alone despite normal tissue T-3 concentrations. Cortex type III 5-deiodinase was only increased when hyperthyroidism was induced by infusion of T-3. Liver type I 5'-deiodinase (DI) paralleled the changes in plasma and tissue T-3 regardless of whether T-4 or T-3 was infused. On the contrary, the increase in lung DI, proportional to the increases in plasma and tissue T-3, was higher when T-4 was infused. As a rule, the tissues with DII presented a tighter homeostasis in their T-3 concentrations than the tissues with DI. In conclusion, the regulation of iodothyronine deiodinases depends on the hormone infused into the thyroidectomized animals and on the tissue in which the deiodinase is studied, demonstrating the existence of tissue-specific regulation of its thyroid hormone concentrations.