We investigated the serum thyroid hormones an the degree of diabetic control in 30 children and adolescents suffering from type 1 insulin-dependent diabetes, first while they were being treated with porcine insulin (group A) and then while 23 of them, random selected, were being treated with human insulin (group C). The purpose of our study was to evaluate if the change to human insulin would result in better metabolic control and disappearance of the tendency to the low T3 syndrome that was evidenced in group A. If we compare group A with group C, it can be seen that the degree of diabetic control is higher in group C (decrease in HbA1C), but that the tendency to the low T3 syndrome has not disappeared. Then we selected inside the groups A and C the patients in good metabolic control, and called them respectively group B and group D. If we compare group B with group D it can be seen that there is not any significant difference at all between them and that both groups show a tendency to the low T3 syndrome if compared with the control group (group K). In conclusion, we may state that substituting porcine with human insulin has not brought about any significant metabolic improvement in our patients, as can be seen from the persistent tendency to the low T3 syndrome.