CONTEXT: Autoimmune thyroid diseases (AITDs) can be associated with type 1 diabetes (DM1). The prevalence of serum antibodies against thyroid hormones (THAb) in subjects with autoimmune diseases other than DM1 is increasing. No data are available for DM1. OBJECTIVE: The objectives were evaluate the rate of associated AITD; the rate of positiveness for serum THAb; the panel of THAb based on thyroid hormone interaction and on Ig class; and the association of AITD alone, THAb alone, or AITD plus THAb with diabetes-related complications. DESIGN: This was an observational, prospective study with 6-year (2005-2011) follow-up. SETTING: The setting was an outpatient diabetes clinic. PATIENTS: Fifty-two consecutive subjects (53.8% males; mean age, 37.4 ± 7.4 y; diabetes duration, 19.9 ± 8.2 y) with DM1. All participants completed the study. MAIN OUTCOME MEASURES: Main outcome measures were AITD rate; THAb positivity according to hormone interaction and Ig class; association of AITD and THAb with diabetes-related complications. RESULTS: AITD rate increased from baseline (34.6%) to follow-up (38.5%). Subjects with DM1 had a high prevalence of THAb (92.3%). The presence of AITD at baseline was associated with subsequent development of macroangiopathy (0 vs 33% at baseline and follow-up, respectively; P = .029). Some THAb patterns, the majority having T3 binding in common, were associated with the progression and development of diabetes-related complications. CONCLUSIONS: THAb synthesis in DM1 might be driven by increased glycosylation of thyroglobulin. Anti T3-THAb may cause a relative "tissue hypothyroidism" by sequestering thyroid hormone, this at least partially contributing to worsening diabetes-related vascular complications. In a clinical setting THAb positivity could identify subjects more likely to develop diabetes complications.