1. Some metabolic indicators of thyroid hormone activity have been investigated in rats fed on either protein-deficient or energy-restricted diets. 2. Rats were divided into three groups. Control animals were maintained on a diet of protein energy: total energy (P:E) value of 0.20, while the low-protein group (LP) were allowed ad lib. access to food of P:E 0.03. Energy-restricted (ER) rats were given limited amounts of a control diet (P: E 0.20) such that their rate of growth matched that of LP

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... animals. 3. Animals fed on the LP diet had elevated plasma concentrations of both total and Free triiodothyronine (T,) concentrations whereas those on the ER regimen showed values below those of controls. 4. The activities of mitochondria1 a-glycerol-3-phosphate dehydrogenase (EC 1 . 1 .99.5) and of the a-glycerol-3-phosphate shuttle system were elevated in the liver of LP rats, but malate-aspartate shuttle operation was reduced. All three activities were reduced in ER animals. 5. Cytochrome c oxidase (EC 1 .9.3.1) activity of brown adipose tissue indicated a high rate of thermogenic activity in this tissue in LP rats, but ER animals showed some evidence of below normal function. 6. The results indicate that both the raised plasma T, of LP rats and the reduced levels observed in ER animals are physiologically significant.