Proposed mechanism for increased thyroxine deiodination in cold-acclimated rats

Groups of male rats were administered isoproterenol (ISO), thyroxine (T4), or both (ISO + T4) daily for 20 days in an attempt to mimic the effect of cold acclimation on the rate of outer-ring deiodination of T4 to triiodothyronine (T3) by 2,000-g supernatants of homogenates of liver and kidney. The rates of hepatic and renal deiodination of T4 to T3 in an additional group of rats exposed to 4 +/- 1 degree C for 20 days were 53 and 71% higher, respectively, than control. Administration of ISO (100 micrograms . kg-1 . day-1) did not affect the rate of deiodination of T4 to T3 by either hepatic or renal tissue. Administration of T4 (50 micrograms . kg-1 . day-1) resulted in rates of hepatic and renal deiodination of T4 that were 297 and 222% higher, respectively, than control. Administration of ISO + T4 resulted in a rate of conversion of T4 to T3 not significantly different from that observed when T4 was administered alone. Serum concentration of T4 was elevated after administration of both T4 and ISO + T4, whereas serum concentration of T3 was elevated significantly above that of control only in the cold-acclimated group. These results suggest that the increased rate of 5'-monodeiodination of T4 by hepatic and renal homogenates from cold-acclimated rats is not a result of increased beta-adrenergic activity but can be accounted for by the increase in thyroid activity observed in these animals.