Effects of starvation on in vivo gluconeogenesis in hypo- and hyperthyroid rats

Studies were performed on rats in vivo to determine whether starvation disrupts glucose metabolic balance after removal of the thyroid gland. Intact controls and thyroidectomized rats were injected daily with 0, 0.1, 1.8, or 25 microgram L-T4/100 g BW. Glucose spaces were similar in all groups. The disappearance of labeled glucose from blood was faster in the thyroidectomized rats injected with 25 microgram L-T4 than in the other groups. Starvation enhanced the production of [14C]glucose from [3-14C]pyruvate in all groups, but this effect occurred earlier in control rats and thyroidectomized rats given 1.8 or 25 microgram L-T4 than in thyroidectomized rats given either 0 or 0.1 microgram L-T4. Starvation also enhanced the appearance of radioactivity in liver glycogen 30 min after the injection of [3-14C]pyruvate in all groups, but this effect was lesser in thyroidectomized rats given 0, 0.1, or 25 microgram L-T4 than in other groups. The normal net production of glucose in fed thyroidectomized rats may be the result of a balanced equilibrium between reduced gluconeogenesis and glycolysis. Results obtained in thyroidectomized rats given 25 microgram L-T4 are discussed in terms of the augmented utilization of the newly formed glucose which compensates for their enhanced gluconeogenesis.