We study 48 compensated diabetic patients, 41.6% of whom were male, 68.75% had diabetic neuropathy and/or vascular complications (macro-microangiopathy). In 60.4% of the patients the study began at ten years of disease evolution. We selected 14 healthy adults as a control group, 50% being male. Plasma levels of T3, T4 and TSH were measured by RIA and the antimicrosomal antibody (AMT) and antithyroglobulin antibody (ATG), detected by passive hemagglutination. A significant decrease in T3 level (p less than 0.05) was found in controlled diabetic patients, compared to T3 levels of the control group; there was a significant reduction in T3 level (p less than 0.05) in diabetic patients with vascular disease and in female diabetics (p less than 0.05), compared to the control group. There were no statistically significant differences in the other groups, including other parameters measured. We concluded that there is a decrease of T3 in all diabetic patients, this being more noticeable in diabetic females and diabetic patients with vascular disease.