The aim of this study was to better understand the frequency of autoimmune thyroid and diabetes antibodies in patients with type 1 diabetes mellitus (T1DM) compared with their siblings. Glutamic acid decarboxylase antibodies (GADA), islet cell antibodies (ICA), insulin autoantibodies (IAA), and thyroid autoantibodies were studied in all subjects. The rates of positive GADA and IAA were significantly higher in probands compared to in siblings (p<0.001) or controls (p<0.001). All pancreatic autoantibodies were not significantly different between the siblings and the healthy controls. Thyroid antiperoxidase antibody (TPOAb) and antithyroglobulin antibody (TGAb) were significantly different between the probands and the control subjects (p=0.002 and p=0.018, respectively). The rates of TPOAb and TGAb positivity in siblings were higher than in those of the controls, but there was no significant difference between the two groups. However, thyroid autoimmunity (TA) was significantly different among the groups (p=0.004). Siblings of the TA-positive probands were shown to have a greater prevalence of thyroid antibodies than did the controls (p=0.022), but siblings of the TA-negative probands did not have such a prevalence compared with the control subjects. The prevalence of pancreatic and thyroid antibodies positivity in probands was statistically significant compared with the siblings and the controls. Siblings of TA-positive probands revealed a greater prevalence of thyroid antibodies than did the controls. Therefore, the screening for TA in siblings, particularly siblings of TA-positive probands, is as important as it is in probands.