Thyroxine deiodination during cold exposure in the rat.

1. The urinary radio-iodide excretion following injection of [(131)I]thyroxine and tri-iodothyronine is increased in acutely cold-exposed, thyroidectomized rats.2. 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-iodide. It is concluded that cold exposure accelerates the rate of deiodination of both thyroxine and tri-iodothyronine.3. It is confirmed that both adrenaline and noradrenaline administered in vivo can enhance the rate of thyroxide deiodination.4. The sympathetic blocking agents guanethidine and bethanidine reduce the extra deiodination of thyroxine induced by cold exposure. Bethanidine was also found to reduce the deiodination of tri-iodothyronine during cold.5. There is a positive correlation between the deiodination of thyroxine and the urinary catecholamine excretion in the cold-exposed but not the warm-exposed rat.6. It is suggested that the enhanced deiodination during cold exposure is mediated by the release of noradrenaline from the sympathetic nervous system.