Treatment of rats with T₃ resulted in a significant decrease in body weight, while the heart weight increased. T₄ treatment had less marked effect on body weights but resulted in decreased heart weights. Serum T₄ levels decreased significantly with simultaneous increase of T₃ level following T₃ treatment, whereas with T₄ treatment, levels of both T₄ and T₃ increased in the serum. Low doses of T₃ (0.5 µg ) caused decrease in mitochondrial protein content while high dose of T₄ (1 µg), caused significant increase in mitochondrial mass. The state 3 respiration rates were significantly depressed following T₃ and T₄ treatments, in a substrate specific manner with the effects being more pronounced with T₃; these responses with T₄ were dose-dependent for succinate and ascorbate + N,N,N′,N′-tetramethyl-p-phenylenedíamme. State 4 respiration rates also exhibited similar corresponding changes. ADP/O ratios were not changed but ADP-phosphorylation rates were decreased significantly particularly so with the T₃-treated animals. Treatment with T₃ also resulted in lowering of intramitochondrial cytochrome contents. Similar effects were seen also with higher doses of T₄. The results thus indicate that T_3- and T_4- thyrotoxicosis results in impaired energy metabolism in heart mitochondria.

Effects of T_3- and T₄-induced thyrotoxicosis on temperature-dependent Arrhenius kinetics of succinate oxidase and Mg²⁺- and Mg²⁺ + 2,4-dinitrophenol-dependent ATPase activities in rat heart mitochondria were examined, For succinate oxidase system, treatment with T₃ and T₄ caused increase in the energy of activation in high temperature range in a dose-dependent manner. For low temperature range, increase in energy of activation was apparent only with higher doses of the hormones; with low doses a small but reproducible decrease was evinced. The phase transition temperature decreased significantly under these conditions. For the Mg²⁺- and Mg²⁺+2,4-dintro-phenol-dependent-ATPase activities, the activation energy values in high temperature range decreased in general. Activation energy values in low temperature range recorded a generalized increase in the Mg²⁺-ATPase enzyme system while the value did not change significantly for the Mg²⁺ + 2,4-dinitrophenol-ATPase; phase transition temperature registered a small but reproducible decrease under these conditions. The results are suggestive of increased membrane fluidization possibly through increased proportion of unsaturated fatty acids. The differential effects seen for succinate oxidase and ATPase systems are consistent with different lipid protein domains of these enzyme systems.