Thyroid hormones in rats during long-term cold exposure and hypometabolic effect of reverse triiodothyronine on adrenaline induced thermogenesis

One-day cold exposure to 5 degrees C induced temporal increase of reverse-triiodothyronine (rT3), triiodothyronine (T3) and thyroxine (T4) concentrations in the blood of rats (by 151%, 91% and 107%, respectively). The levels of all thyroid hormones remain elevated for at least 10 days of cold exposure. The injection of rT3 (330 micrograms kg-1 s.c.) inhibited the resting and adrenaline stimulated metabolism 1-3 days after application independently from the thyroid status of the animals. Since the hypothermic effect of rT3 occurred in thyroidectomized rats, it was concluded that rT3 acted primarily on peripheral, tissues, rather than on the thyroid.