A 45-year-old woman presents with a 6-month history of fatigue and swelling in her neck. Physical examination shows a goiter. A CBC discloses megaloblastic anemia and a normal reticulocyte count. Additionally, there is an elevated serum level of TSH and antithyroid antibodies. Needle aspiration of the left lobe of the thyroid reveals benign follicular cells and numerous lymphocytes. Anemia in this patient is most likely caused by antibodies directed to which of the following targets?

This patient has chronic lymphocytic thyroiditis (Hashimoto thyroiditis) and pernicious anemia. Pernicious anemia is a megaloblastic anemia that is caused by malabsorption of vitamin B12 due to a deficiency of the intrinsic factor. In many cases, pernicious anemia is associated with other autoimmune diseases (e.g., Hashimoto thyroiditis, Graves disease, Addison disease, or diabetes mellitus type 1). Circulating antibodies to parietal cells, some of which are cytotoxic in the presence of complement, occur in 90% of patients with pernicious anemia. Two thirds of patients display an antibody to the intrinsic factor that prevents its combination with vitamin B12 thereby preventing formation of the complex that is later absorbed in the ileum. Half of all patients with pernicious anemia have circulating antibodies to thyroid tissue.Diagnosis: Pernicious anemia, Hashimoto thyroiditis