Fasting and hypocaloric diets are known to induce a reduction of triiodothyronine (T3) and to increase reverse triiodothyronine (rT3) in normal and obese subjects. The effect of 8-day fasting was evaluated on T3, thyroxine (T4), free T4, rT3, TSH, immunoreactive insulin (IRI), thyroxine binding globulin (TBG) and glycemia in 21 obese subjects (5 males, 16 females) grouped according to the average starting blood glucose concentration in: group I, diabetic obese subjects (9 patients); group II, non diabetic obese subjects (12 patients). All patients had no history of recent weight loss due to dietetic therapy or drugs. Blood samples were drawn in the morning at 0, 2, 4, 6, 8 days after total caloric deprivation and 2 days after refeeding. A superimposable variation of weight and glucose concentration were seen in both groups. In addition, no difference was observed in ketone body excretion, SH, TBG, T4, free T4. In group II a significant decrease of IRI was observed during diet (p less than 0.05); T3 decreased (p less than 0.01) and rT3 increased (p less than 0.01) significantly. No variations in T3 and rT3 values were observed in group I. These results are consistent with a possible role of glucose metabolism in the genesis of the low T3 syndrome.