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The single intraperitoneal injection of a labelled glucose load has been shown to maintain a steady state with respect to the specific activities of blood glucose and expired CO2for extended periods of time in hyperthyroid and hypothyroid rats. Measurements were made of rates of oxidation of glucose-1-C14and glucose-6-C14during the steady state.The results indicate that glucose oxidation by way of the glycolytic pathway is markedly elevated in the hyperthyroid rat and parallels the increased rate of CO2expiration. Glucose oxidation by way of the hexose monophosphate pathway is completely suppressed. This effect is considered to be due to the preferential maintenance of diphosphopyridine nucleotide in the reduced form.In the hypothyroid rat a decreased rate of glucose oxidation by way of both oxidative pathways parallels the decreased rate of CO2production.The results support the view that the primary action of thyroxine is on oxidative activity. The observed effect on the hexose monophosphate pathway appears to be secondary.

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