In vitro hepatic deiodination of L-thyroxine to 3,5,3'-triiodothyronine in cold-acclimated rats.

An in vitro model has been employed to evaluate the effect of cold acclimation on peripheral outer ring deiodination of thyroxine (T4) to triiodothyronine (T3) by 9,000 g supernatants of fresh liver homogenates. Hepatic T3 generation from T4 in male rats exposed to 4 +/- 1 degree C for 12 wk was 41 and 24% higher than control after 30- and 60-min incubation periods, respectively. This was associated with a 49% increase in serum T3 concentration in the cold-acclimated rats, whereas serum T4 concentration did not differ from the control. Thus, hepatic deiodination of T4 appears to serve as an important source for production of T3 during cold acclimation. It is suggested that the increased beta-adrenergic activity observed previously in cold-acclimated rats stimulates this change.