Lipolytic and Ketogenic Fluxes in Human Hyperthyroidism*

The effect of hyperthyroidism on lipolytic and ketogenic fluxes was determined by measuring simultaneously (stable isotope methodology) glycerol, nonesterified fatty acids (NEFA), and ketone body (KB) kinetics in euthyroid and hyperthyroid subjects. In the postabsorptive state hyperthyroid patients had normal concentrations of insulin and glucagon, but increased concentrations (P less than 0.01) and turnover rates (P less than 0.01) of glycerol, NEFA, and KB. The ratio of NEFA appearance rate to glycerol appearance rate was decreased in hyperthyroid subjects (2.34 +/- 0.23 vs. 3.15 +/- 0.22; P less than 0.05), indicating that intracellular cycling between triglycerides and fatty acids was increased. The percentage of NEFA flux used for KB production, calculated from NEFA disappearance rates and KB appearance rates, was increased in hyperthyroid patients (21.20 +/- 2.75% vs. 13.37 +/- 0.63%; P less than 0.05), suggesting a diversion during hyperthyroidism of hepatic fatty acid metabolism toward ketogenesis. However, when the plasma NEFA levels of control subjects were raised by the infusion of a triglyceride emulsion to levels comparable to those observed in hyperthyroid patients their percentage of NEFA flux used for ketogenesis rose to values slightly higher (26.30%) than those of hyperthyroid subjects. In conclusion, 1) hyperthyroidism results not only in increased lipolysis, but also in enhanced triglyceride-fatty acid cycling, which could contribute to the excessive energy expenditure; and 2) the increased KB production of hyperthyroid patients results more from an increase in NEFA availability than from a direct stimulation of hepatic ketogenesis.