The role of L-3,3'-5 triiodothyronine (T3), in a pathophysiological range, on gluconeogenesis from low concentration of glycerol (2 mmol/L), was investigated in isolated liver cells from 24-hour fasted rats either thyroidectomized, normal, or treated by a T3 dose ranging from 1, 5, or 10 micrograms/d/100 g body weight (BW) during 3 days to 50 micrograms during 7 days. Gluconeogenesis from glycerol was decreased by 63% in hypothyroid rats and increased by 35% in severely hyperthyroid rats. However, in cells from mild hyperthyroid rats no increase of gluconeogenesis was observed. Nevertheless, in mild hyperthyroidism, alpha-glycerophosphate (G3P) was significantly decreased and gluconeogenesis from glycerol was not inhibited by the addition of ethanol (10 mmol/L), both of which have a drastic effect in cells from thyroidectomized rats. The decrease of gluconeogenesis observed in cells from thyroidectomized rats was reversed by the addition of pyruvate (10 mmol/L). Thus, when the cells were in a "reduced state" (addition of ethanol) the differences between the group were magnified, and when the cells were in an "oxidized state" (addition of pyruvate) the differences were suppressed. These findings suggest that alteration of the capacity of reducing equivalents transfer from the cytoplasmic compartment to the mitochondria is the main mechanism by which mild hyperthyroidism can stimulate gluconeogenesis.