Aim of the studyThe primary aim of the study was to evaluate the differences in metabolic control and chronic microvascular complications in patients with type 3 autoimmune polyglandular syndrome (APS3), compared to type 1 diabetes mellitus (T1DM) alone. Secondary aims were to evaluate the age of autoimmune thyroid disease (AIT) onset and the effects of levothyroxine treatment on metabolic control in patients with APS3. Material and methodsWe retrospectively reviewed 276 patients with T1DM alone and 214 patients with APS3 and evaluated clinical and metabolic parameters and microvascular complications. ResultsPatients with T1DM showed a longer duration of diabetes (p = 0.001) and lower age of diabetes onset (p = 0.020) compared to patients with APS3. Female gender (p = 0.001) and microalbuminuria (p = 0.006) were significantly more frequent in patients with APS3 compared to T1DM. In addition, patients with APS3 showed higher AIT onset frequency in the 16–30 quartile age-range. Furthermore, APS3 patients treated with levothyroxine showed significantly better HbA1c values than non-treated patients (p = 0.001). ConclusionsWe found that patients with APS3 showed positive microalbuminuria, earlier than T1DM. Patients with APS3 showed higher frequency of AIT age of onset in the 16–30 age-range and those treated with levothyroxine had better metabolic control, than untreated ones.