In order to determine the endocrine and metabolic state of thyrotoxicosis we measured blood glucose and plasma insulin response to ingestion of a mixed meal in 19 euthyroid and 9 hyperthyroid subjects. Moreover concentrations of glucose, free fatty acids (FFA), glycerol, acetoacetate (AcAc) beta-hydroxybutyrate (beta-OHB), insulin and human growth hormone (hGH) were determined in the blood of both healthy and hyperthyroid patients after an overnight and a 39-h fast. In another group of thyrotoxics the overnight fasting respiratory quotient (RQ) was measured. After a mixed meal blood glucose and plasma insulin changes of FFA, AcAc and beta-OHB was significantly higher in thyrotoxics, whereas hGH increase did not appear significantly greater in these subjects. There was no statistical difference between the respiratory quotient mean values found in hyperthyroid and in control subjects. In conclusion, these data indicate that in thyrotoxicosis absolute insulin response to a mixed meal is normal and that food deprivation considerably increase lipid mobilization from adipose tissue and causes an exaggerated starvation ketosis. The RQ mean valoue suggests that in the hyperthyroid state lipid-derived fuel as well as carbohydrate-derived fuel contributes to the increased oxygen consumption.