1. Glucose turnover was examined by using simultaneous bolus intravenous tracer administration of both [3-3H]glucose and [1-14C]glucose in hyperthyroid and hypothyroid subjects before and during appropriate therapy and in matched controls.
2. In hyperthyroid subjects, total glucose turnover (as indicated by analysis of decay of [3-3H]glucose specific radioactivity) was increased (18.1 ± 1.0 vs 12.4 ± 0.9 μnol kg−1 min−1, P < 0.01), as was the metabolic clearance rate for glucose (3.5 ± 0.3 vs 2.4 ± 0.2 ml kg−1 min−1, P < 0.01).
3. Glucose turnover rates determined with [1-14C]glucose as tracer were lower than total glucose turnover rates in all subjects. This difference represents an index of recycling of the 14C label between glucose and glucose-derived C3 intermediates.
4. The recycling index was increased in hyperthyroidism both in amount (4.7 ± 0.6 vs 1.3 ± 0.3 μmol kg−1 min−1, P < 0.001) and when expressed as a percentage of total glucose turnover (27 ± 3 vs 10 ± 2%, P < 0.001).
5. Turnover rates determined by [1-14C]glucose represent irreversible glucose disposal and were similar in hyperthyroid and control subjects (hyperthyroid 13.4 ± 1.1 vs control 11.2 ± 0.9 μmol kg−1 min−1). The major fraction of the increase in total glucose turnover could thus be accounted for by increased cycling of glucose.
6. Total glucose turnover, metabolic clearance and indices of recycling returned to normal with treatment of hyperthyroidism.
7. In hypothyroidism, total glucose turnover (hypothyroid 8.2 ± 0.3 vs control 11.7 ± 1.2 μmol kg−1 min−1, P < 0.01) and metabolic clearance (hypothyroid 1.6 ± 0.1 vs control 2.3 ± 0.2 ml kg−1 min−1, P < 0.001) were decreased and returned to normal with thyroxine therapy. Indices of glucose recycling were normal.