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?