Background: The prevalence of both islet cell and adrenal autoimmunity among Asian Indian hypothyroidism patients with Hashimoto's thyroiditis (HT) is lacking in literature. Objectives: The objective of this study was to assess the proportion of Addison's disease (AD) and type 1 diabetes mellitus (T1DM) in patients with HT. Materials and Methods: The patients with hypothyroidism due to HT were included in this study over 2 years. Primary hypothyroidism was defined as high serum thyroid-stimulating hormone (>5.5 mIU/L) with or without low thyroxine level. HT was defined by the presence of high thyroid peroxidase antibody (Ab) titer (>35 IU/ml). Autoimmune markers of AD and T1DM, i.e., adrenal (21-hydroxylase) Ab, glutamic acid decarboxylase (GAD) Ab, and insulinoma-associated antigen-2 (IA-2) Ab were measured among them. In addition, 250 μg adrenocorticotropic hormone (ACTH) stimulation test was done in patients with adrenal Ab. Similarly, beta cell function was assessed in patients with GAD and/or IA-2 Ab. Results: Out of 150 patients screened, 136 patients were included in this study. Seven patients had adrenal Ab while 15 had IA-2 Ab. The GAD Ab was not present in any of the patients in the study. ACTH stimulation test was done in four of seven patients with adrenal Ab and beta cell function was assessed in 8 of 15 patients with islet cell Ab. All patients with adrenal Ab had normal adrenal function and 1 out of 15 with IA-2 Ab developed diabetes mellitus during follow-up. Conclusions: Either adrenal or islet cell Ab was found in 16% Asian Indian hypothyroidism patients with HT.