Thyroxine deiodination during cold exposure in the rat

Abstract

The urinary radio-I excretion following injection of [131I]T4 and tri-iodothyronine [T3] is increased in acutely cold-exposed, thyroidectomized rats. This cold response is demonstrable in animals which are fed or fasted or deprived of both food and water. Cold exposure does not increase the rate of urinary excretion of a tracer dose of radio-I. Cold exposure accelerates the rate of deiodination of both T4 and T3. Both adrenaline [epinephrine] and noradrenaline [norepinephrine] administered in vivo can enhance the rate of T4 deiodination. The sympathetic blocking agents guanethidine and bethanidine reduce the extra deiodination of T4 induced by cold exposure. Bethanidine also reduced the deiodination of T3 during cold. There is a positive correlation between the deiodination of T4 and the urinary catecholamine excretion in the cold-exposed but not the warm-exposed rat. The enhanced deiodination during cold exposure is mediated by the release of norepinephrine from the sympathetic nervous system.