Fecal excretion of thyroxine in warm- and cold-acclimated rats

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Abstract
The daily excretion rate of fecal thyroxine was determined in warm- or cold-acclimated rats. Fecal thyroxine and dietary thyroxine were measured directly by a competitive protein binding analysis technique. An analysis of the Master laboratory chow diet confirmed that one-third of the iodine content was in the form of thyroxine. When fed this diet, cold-acclimated rats ingested 70% more thyroxine and excreted 75% more thyroxine than warm-acclimated rats. On the other hand, rats fed a thyroxine-free diet supplemented with iodine in the drinking water, while showing no significant increase in fecal excretion of thyroxine upon cold acclimation, developed a similar degree of cold resistance as those fed the Master laboratory chow containing 0.58 μg/g of thyroxine. Rats on a thyroxine-free diet showed a significantly lower fecal thyroxine excretion rate at both temperatures than did those on the Master laboratory chow. The results indicate that the greater fecal clearance of thyroxine usually found in cold-acclimated rats is not an essential feature of cold acclimation and depends either on the increased fecal bulk or the increased ingestion of dietary thyroxine.