Type 1 diabetes mellitus (T1DM) is characterized by selective autoimmune destruction of pancreatic b-cells, resulting in insulin deficiency. Associated autoimmune disorders, such as celiac disease, autoimmune thyroiditis, and gastritis, can coexist in patients with T1DM. These disorders are characterized by the presence of antibodies against tissue transglutaminase (anti-tTG-IgA), thyroglobulin, and thyroid peroxidase (anti-TG, anti-TPO), as well as antibodies against gastric parietal cells. Children with T1DM may also develop organ-specific multiple autoimmunity, with the coexistence of one or more autoimmune disorders. Furthermore, there is a lot of controversy regarding the role of thyroid autoimmunity in the pathogenesis of thyroid cancer. We present a child with T1DM and multiple autoimmunity including autoimmune Hashimoto's thyroiditis (HT), who developed thyroid cancer. The literature on the prevalence of associated autoimmunity in children with T1DM and the prevalence, pathogenesis, and timely diagnosis of thyroid cancer among patients with HT is also reviewed.