Thyroid diseases are one of the most common metabolic disorders in the human population. In this work, we present data concerning changes in the activity and kinetic parameters of several enzymes associated with both anabolic (glucose-6-phosphate dehydrogenase-G6PDH, EC 1.1.1.49; 6-phosphogluconate dehydrogenase-6PGDH, EC 1.1.1.44; malic enzyme-ME, EC 1.1.1.40; and isocitrate dehydrogenase-IDH, EC 1.1.1.42) and catabolic (NAD-dependent malate dehydrogenase-NAD-MDH, EC 1.1.1.37; and lactate dehydrogenase-LDH, EC 1.1.1.27) processes under conditions of hypothyroidism and T(3) treatment. Hypothyroidism was induced in rats by the surgical removal of the thyroid gland. T(3)-treated rats were injected by T(3) (0.5 mg T(3)/kg body weight daily during 10 days). We have found that T(3) treatment caused an increase of NAD-MDH activity as well as heart hypertrophy whereas the activity of LDH increased in the direction of pyruvate reduction. Moreover, we observed increased activity of both enzymes in the liver. These results confirm earlier observation concerning the relevance of oxidative metabolism in the heart under T(3) treatment. Hypothyroidism resulted in changes in the LDH activity in the heart whereas NAD-MDH activity did not change. Moreover, our data show that T(3) treatment caused an increase of G6PDH, 6PGDH, and ME activities in the liver. We also observed a decrease of IDH activity in both organs, whereas hypothyroidism caused the opposite effect. This data indicate that either deficiency or excess of thyroid hormones can prove to be particularly dangerous for the physiology of the heart muscle by disturbing bioenergetic and anabolic processes.