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

Abstract

The biliary-fecal route of T4 excretion was examined by means of T4 labelled with 131I in the thyrpidectomized, T4-maintained rat during cold exposure. The rate of excretion of T4 into the feces is raised in rats exposed to cold for 2 weeks and fed ad libitum on a high roughage diet. When the fecal volumes of warm- and cold-exposed animals are made similar by controlling the roughage content of the diet the difference in their fecal T4 excretion rates is much reduced. Warm- and cold-exposed rats eating the same amount of food excrete T4 into their feces at similar rates. Even when warm-and cold-exposed rats excrete T4 into their feces at the same rate the cold- exposed animals still show a shorter half-life for blood T4. An acceleration of the rate of loss of T4 from the blood is demonstrable in fasting animals within 10 hr. of exposure to cold. Ligation of the bile-duct does not affect this acute response to cold. The amount of T4 lost into the bile within the 1st 8 hr. after intravenous injection is similar in warm- and cold-exposed animals. The clearance of T4 into the bile is, however, greater in the cold-exposed animal since the liver is working against a lower level of blood T4. An acceleration of the biliary-fecal route of T4 excretion is not the only process at work tending to reduce the biological half-life of T4 during cold exposure.