Thyroid hormone regulation of glucose homeostasis in the miniature pig

The effect of different thyroid states on glucose homeostasis was investigated during metabolic adaptation to starvation in the conscious unrestrained miniature pig. Moderate hyperthyroidism increased the rate of glucose turnover, whereas hypothyroidism was without effect. Glucose recycling was elevated in hyperthyroid pigs, and reduced after thyroidectomy. Supplementary doses of T4 normalized total glucose recycling. Glucose metabolic clearance rate and pool size were unaffected by thyroid hormones. During starvation serum insulin showed a similar decrease in all thyroid states; glucagon increased in euthyroid and hypothyroid pigs, although it was already elevated in the hyperthyroid fed state. Serum cortisol levels although varying were enhanced in hyperthyroid and hypothyroid-T4-treated pigs. Glucogenic precursor concentration and cumulative urinary N-excretion were increased in hyperthyroid pigs. It is concluded that 1) even a moderate hyperthyroidism produces an increase in glucose turnover and a concomitant acceleration in protein breakdown, and 2) thyroid hormone is essential for the starvation-induced total glucose recycling.