The biliary-faecal excretion of thyroxine during cold exposure in the rat.

1. The biliary-faecal route of thyroxine excretion has been examined by means of thyroxine labelled with (131)I in the thyroidectomized, thyroxine-maintained rat during cold exposure.2. The rate of excretion of thyroxine into the faeces is raised in rats exposed to cold for 2 weeks and fed ad libitum on a high roughage diet.3. When the faecal volumes of warm- and cold-exposed animals are made similar by controlling the roughage content of the diet the difference in their faecal thyroxine excretion rates is much reduced.4. Warm- and cold-exposed rats eating the same amount of food excrete thyroxine into their faeces at similar rates.5. Even when warm- and cold-exposed rats excrete thyroxine into their faeces at the same rate the cold-exposed animals still show a shorter half-life for blood thyroxine.6. An acceleration of the rate of loss of thyroxine from the blood is demonstrable in fasting animals within 10 hr of exposure to cold.7. Ligation of the bile-duct does not affect this acute response to cold.8. The amount of thyroxine lost into the bile within the first 8 hr after intravenous injection is similar in warm- and cold-exposed animals. The clearance of thyroxine into the bile is however greater in the cold-exposed animal since the liver is working against a lower level of blood thyroxine.9. An acceleration of the biliary-faecal route of thyroxine excretion is not the only process at work tending to reduce the biological half-life of thyroxine during cold exposure.