Abstract: The frequency and functionality of peripheral blood invariant (iNKT) cells and their subsets, as well as other regulatory T-cell subsets, were evaluated in patients with type 1A diabetes mellitus (DM1), Hashimoto''s disease, and Graves'' disease. In addition to healthy individuals (HC), patients with type 2 diabetes mellitus (DM2) were included as controls because this disease has a different physiopathology. A similar frequency of total iNKT cells, as well as their subsets, existed among HC and the different study groups. Similar results were reported when we compared the frequency of CD4+/CD25high T cells, CD8+/CD28negative T cells, and γ-δ T cells among HC and study groups, whereas patients with DM2 exhibited a higher frequency of CD8+/CD28negative T cells compared with HC and DM1. Also, patients with DM2 exhibited a lower frequency of CD4negative and CD4+ iNKT cells expressing tumor necrosis factor-α (TNF-α) than HC. We did not observe significant differences in the frequency of iNKT cells expressing interleukin-4 or interferon-γ among study groups and controls. Our findings support a normal frequency and function of peripheral blood iNKT cells in different endocrine autoimmune diseases, but an abnormal expression of TNF-α by circulating iNKT cells from patients with DM2. [Copyright &y& Elsevier]