Thyroid antibodies (anti-TPO, anti-TgAb)

Anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (TgAb) antibodies target intrathyroidal enzymes and are the principal serologic markers of autoimmune thyroid disease. They are measured by chemiluminescent or electrochemiluminescent immunoassay with assay-specific cutoffs (commonly anti-TPO <34 IU/mL and TgAb <115 IU/mL); results should always be interpreted with the manufacturer's reference range. Anti-TPO is positive in roughly 90% of Hashimoto's thyroiditis and approximately 75% of Graves' disease; TgAb is less specific but useful in differentiated thyroid cancer monitoring as it can interfere with thyroglobulin measurement. Per the 2017 ATA pregnancy guidelines, anti-TPO positivity increases the risk of miscarriage, preterm birth and postpartum thyroiditis, and influences treatment thresholds in subclinical hypothyroidism. Major confounders are assay heterogeneity, intercurrent viral infection, and other autoimmune conditions (type 1 diabetes, vitiligo) raising titres without thyroid dysfunction.