Critical studies on determination of thyroid secretion rate in cold-adapted animals

¹ U.S. Naval Radiological Defense Laboratory, San Francisco, and National Research Council, Ottawa, Canada

Estimation of thyroid secretion rate by determining the amount of l-thyroxine required to block the thyroidal I¹³¹ release, confirmed that the thyroxine requirement of 6 C-acclimated rats is twice as high as that of 23 C-acclimated controls (5.5 ± 0.7 µg/100 g per day against 2.73 ± 0.3 µg/100 g per day). In white rats acclimatized to cold by outdoor exposure during the winter on the other hand, it was only 1.8 ± 0.38 µg/100 g per day. Larger amounts of thyroxine were found in the feces of the 6 C-acclimated rats (1.66 ± 0.66 µg/100 g body wt per day) as compared to 0.69 ± 0.07 µg/100 g body wt per day for controls. Urinary elimination rates of I¹³¹ were more rapid in "6 C rats" than in controls. These observations indicate that the greater requirement for thyroxine in cold-adapted rats is at least partly due to greater fecal loss of the hormone. It seems plausible to seek an explanation for the lesser thyroxine requirement of the outdoor winter rats in their fecal elimination rate of that hormone.

Submitted on June 18, 1964

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