What does the Thyroid Peroxidase Antibody (Anti-TPO, Anti-Microsomal Antibody) test measure?

Why is Thyroid Peroxidase Antibody (Anti-TPO, Anti-Microsomal Antibody) measured?

Anti-TPO is the most sensitive and clinically prevalent autoantibody in autoimmune thyroid disease — detected in over 95% of Hashimoto's thyroiditis cases, approximately 75–85% of Graves' disease cases, and approximately 10–15% of the general population without overt thyroid dysfunction. As the most discriminating single serological marker for thyroid autoimmunity, Anti-TPO has direct clinical implications across four domains: (1) aetiological characterisation — a positive Anti-TPO in a patient with hypothyroidism or subclinical hypothyroidism confirms Hashimoto's thyroiditis as the mechanism and predicts the future trajectory; (2) risk stratification — Anti-TPO positivity in a euthyroid patient confers a 2–5% annual progression risk to overt hypothyroidism, rising substantially when baseline TSH is above 2.5 mIU/L; (3) treatment decisions — Anti-TPO positivity in subclinical hypothyroidism (TSH 4.5–10 mIU/L) strengthens the clinical case for levothyroxine initiation and shifts guidelines from 'monitor' to 'consider treating'; and (4) pregnancy management — Anti-TPO positivity is associated with significantly increased risk of gestational hypothyroidism, miscarriage, preterm birth, and post-partum thyroiditis, mandating more aggressive TSH monitoring and lower intervention thresholds throughout pregnancy and the post-partum period.

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 as the upper limit of normal; others use <16 IU/mL or platform-specific cut-offs. Confirm whether Access Medical Laboratories designates a borderline/equivocal range on the report or reports only as positive/negative, and confirm the specific assay platform and methodology (electrochemiluminescence immunoassay — ECLIA, or enzyme-linked immunosorbent assay — ELISA).

Functional medicine optimal

Negative (<34 IU/mL) is the clinical target — the presence of any detectable Anti-TPO at titre above the positivity threshold indicates active thyroid autoimmunity. In functional medicine, some practitioners monitor longitudinal titre trends as a surrogate marker of autoimmune activity and therapeutic response to gluten elimination, selenium supplementation, or immune modulation protocols, though titre reduction is not a validated surrogate endpoint in mainstream endocrinology guidelines. # REVIEWER FLAG: Monitoring Anti-TPO titres as a therapeutic response marker is a functional medicine application not endorsed by mainstream ATA or AACE guidelines — reviewer to confirm appropriate framing.

Units

IU/mL

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