Anti-TPO Ab (RDL)

The Anti‑TPO Ab (RDL) test, also called the thyroid peroxidase antibody or TPOAb assay and historically referred to as anti‑thyroid microsomal antibodies, aids in evaluating autoimmune thyroid disease, including Hashimoto thyroiditis, Graves disease, postpartum thyroiditis, and chronic autoimmune thyroiditis. When interpreted with thyroid function tests, it helps distinguish autoimmune causes of goiter or thyroid dysfunction from nonautoimmune etiologies. Anti‑TPO positivity in the context of hypothyroidism supports a diagnosis of Hashimoto thyroiditis, though antibodies may be present in other thyroid disorders and in patients with systemic autoimmune disease. Testing is used to assess risk of thyroid dysfunction during pregnancy and in newborns of anti‑TPO–positive mothers and may be repeated to follow autoimmune thyroid disease over time.

This assay detects autoantibodies directed against thyroid peroxidase, an apical membrane enzyme that catalyzes iodide oxidation, organification (iodination) of thyroglobulin, and coupling of iodotyrosines to generate T4 and T3. Loss of immune tolerance leads to anti‑TPO production and lymphocytic thyroiditis, with tissue injury and disruption of hormone synthesis that commonly produces hypothyroidism or, less often, a transient thyrotoxic phase. Contemporary sensitive immunoassays identify anti‑TPO in approximately 95% of Hashimoto thyroiditis and about 85% of Graves disease. Antibodies are also observed in idiopathic hypothyroidism, nodular thyroid disease, and thyroid carcinoma, and in patients with systemic autoimmune disorders. Maternal anti‑TPO crosses the placenta and can affect fetal and neonatal thyroid function; therefore, antibody positivity during pregnancy has implications for obstetric and neonatal management.