Effect of high- and low-bulk diets on the thyroxine turnover rate in rats with acute and chronic exposure to different temperatures

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Abstract
Thyroid activity was estimated from thyroxine (T4) turnover rate values by using the single injection isotope dilution technique in rats acclimated to 28 °C and 6 °C and fed either a high-bulk diet (Master Laboratory Cubes (MLC)) or a low-bulk thyroxine-free diet (T4F). In cold-acclimated rats, whether fed MLC or T4F, the metabolic clearance rate of thyroxine was faster than in the 28 °C control rats. The plasma thyroxine level was unchanged after cold acclimation in rats fed MLC but was significantly lower in the T4F-fed rats. The product of the metabolic clearance rate and the plasma thyroxine level or the thyroxine turnover rate was significantly higher after cold acclimation in rats fed the commercial chow diet (MLC) than in rats fed the semipurified T4F diet. This result demonstrates that the increased thyroxine turnover rate, previously observed in cold-acclimated rats, can be taken as another example of a diet-dependent change similar to the thyroid hypertrophy, higher resting metabolic rate, and greater fecal excretion of thyroxine, which have all been shown to be diet-dependent and unnecessary once cold acclimation is established.

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