Fructose-6-phosphate cycling and the pentose cycle in hyperthyroidism.

Hepatic fructose-6-phosphate (fructose-6-P) cycling and pentose cycle activity were quantified in hyperthyroid patients. A measure of the fructose-6-P cycle was the incorporation of 14C, on administering [3-3H,6-14C]galactose, into carbon 1 of blood glucose and the 3H/14C ratio in blood glucose. The measure of the pentose cycle was the randomization of 14C to carbon 1 of blood glucose on administering [2-14C]galactose. [2-3H]Galactose was also administered, so the 3H/14C ratio in blood glucose measured the extent of equilibration of glucose-6-P with fructose-6-P. Patients given [3-3H,6-14C]galactose were restudied when euthyroid. Of the 14C from [3-3H,6-14C]galactose, 7.7-9.5% was in carbon 1 of glucose in both states. 3H/14C ratios were also the same in both states. Fructose-6-P cycling was estimated to be 13 +/- 1% the rate of glucose turnover in the euthyroid and 15 +/- 1% that in the hyperthyroid state. The pentose cycle contributed about 2% to glucose utilization, similar to previous estimates in healthy humans. As in healthy individuals, about 25% of 3H was retained in the conversion of [2-3H]glucose-6-P to glucose. Thus, the fractions of glucose turnover participating in hepatic fructose-6-P and pentose cycling are similar in hyperthyroid and healthy subjects. As a result, augmented fructose-6-P cycling does not substantially contribute to increased hepatic oxygen consumption in hyperthyroidism.