Evidence that thyroid hormones regulate gluconeogenesis from glycerol in man.
We have previously reported that glucose production assessed using radioisotopic methods is 50% increased in hyperthyroidism but 30% decreased in hypothyroidism. These studies, however, do not distinguish between glycogenolysis and gluconeogenesis. In fasting man more than 80% of circulating glycerol is cleared by the liver and enters the gluconeogenic pathway. We have therefore measured glycerol clearance following bolus intravenous glycerol administration as an indirect assessment of gluconeogenic capacity. Hyperthyroid and hypothyroid subjects were compared with separate matched controls after an overnight fast. In hyperthyroid subjects blood glucose and blood glycerol were increased but lactate, pyruvate, and alanine concentrations were normal. Glycerol clearance was increased in hyperthyroidism and followed a double exponential decay with a shortened second component half-time. Endogenous glycerol production was increased three-fold. In hypothyroidism fasting circulating levels of glucose, lactate, pyruvate, alanine, and glycerol were normal but glycerol clearance was diminished. Both first and second component half-times were prolonged in hypothyroidism and endogenous glycerol production was decreased by 50%. Thus in hyperthyroidism glycerol clearance is greatly enhanced whilst in hypothyroidism glycerol clearance is diminished. The magnitude of the changes suggests that alterations in gluconeogenesis are probably the major factors concerned in the reported increase and decrease in glucose production in hyperthyroidism and hypothyroidism respectively.