What does the Thyroid Peroxidase Antibodies test measure?

Why is Thyroid Peroxidase Antibodies measured?

Anti-TPO is the most sensitive and prevalent autoantibody in autoimmune thyroid disease — present in over 95% of Hashimoto's thyroiditis cases and approximately 75–85% of Graves' disease cases. A positive Anti-TPO in a euthyroid patient indicates the presence of thyroid autoimmunity and confers a significantly elevated risk of progression to clinical hypothyroidism — approximately 2–5% per year with a normal TSH, rising substantially with TSH above 2.5 mIU/L. Anti-TPO positivity in a patient with subclinical hypothyroidism (elevated TSH with normal Free T4) is an indication for levothyroxine therapy per ATA and AACE guidelines when TSH is above 10 mIU/L, and is a stronger argument for treatment when TSH is in the 4.5–10 mIU/L range. In pregnancy, Anti-TPO positivity is associated with increased risk of post-partum thyroiditis, miscarriage, and gestational hypothyroidism.

Reference ranges

Standard reference

<34 IU/mL (negative); 34–100 IU/mL (borderline); >100 IU/mL (positive) # REVIEWER FLAG: Confirm Access Medical Laboratories reference range and positivity cut-off for Anti-TPO — thresholds vary between immunoassay platforms. Some laboratories use <35 IU/mL; confirm Access Medical Laboratories' specific cut-off and whether they report borderline/equivocal ranges.

Functional medicine optimal

Not applicable — Anti-TPO is a qualitative autoimmunity marker; negative (<34 IU/mL) is the target. Serial quantitative monitoring of Anti-TPO titres has limited clinical utility as titres do not consistently predict disease progression rate or treatment response.

Units

IU/mL

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.