TPO and hypothyroidism: It should be noted, however, that although positivity for TPO-Ab suggests that hypothyroidism may be present, the clinical definition of hypothyroidism is based on thyroid-stimulating hormone (TSH) concentrations above the reference range and free thyroxine concentrations below the reference range; positivity for TPO-Ab together with clinical laboratory findings of elevated TSH and decrease thyroxine levels would be consistent with the diagnosis of autoimmune primary hypothyroidism [22].