Thyroid Peroxidase Antibodies (Anti-TPO), Serum
The Thyroid Peroxidase Antibody (Anti-TPO) test detects and quantifies IgG autoantibodies against thyroid peroxidase in serum, providing the primary serological confirmation of thyroid autoimmunity in Hashimoto's thyroiditis and Graves' disease.
1
Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — fasting is not required for Anti-TPO measurement. However, biotin supplementation must be withheld for a minimum of 48–72 hours before collection to prevent assay interference.
Fasting
What is the Anti-TPO test?
The Thyroid Peroxidase Antibody (Anti-TPO) test detects and quantifies IgG autoantibodies against thyroid peroxidase in serum, providing the primary serological confirmation of thyroid autoimmunity — the underlying immunological mechanism in the majority of thyroid dysfunction encountered in clinical practice. In iodine-replete populations, autoimmune thyroid disease accounts for over 90% of hypothyroidism and is the pathophysiological basis of Hashimoto's thyroiditis — the most prevalent organ-specific autoimmune disease. Anti-TPO testing transforms a thyroid function result from a biochemical description (the TSH and Free T4 pattern) into an aetiological diagnosis, which carries direct implications for disease counselling, monitoring frequency, treatment decisions, and long-term prognostication. Anti-TPO is ordered as a standalone test when the clinical question is specifically thyroid autoimmune status — for example, characterising the aetiology of a confirmed TSH abnormality, assessing autoimmune progression risk in a euthyroid patient with thyroid-related symptoms, or evaluating reproductive risk in a patient planning pregnancy. It is also routinely included as a component of the Thyroid Panel Comprehensive (BML-THY-020) when full-spectrum thyroid characterisation is required alongside TSH, free thyroid hormones, and TSI. As a standalone order from Access Medical Laboratories, Anti-TPO provides the autoimmune dimension of thyroid assessment efficiently in patients where the TSH and Free T4 picture is already established and antibody characterisation is the remaining clinical question.
What does the Anti-TPO test measure?
Thyroid peroxidase (TPO) is a haem-containing enzyme anchored to the apical membrane of thyroid follicular cells, facing the follicular lumen. It catalyses both the iodination of thyroglobulin tyrosine residues (organification of iodide) and the subsequent oxidative coupling of iodotyrosines to form thyroxine (T4) and triiodothyronine (T3) — making it the enzymatic nexus of thyroid hormone biosynthesis. Anti-TPO antibodies are IgG autoantibodies directed against conformational and linear epitopes predominantly within the immunodominant region (IDR) of the extracellular TPO protein, generated by thyroid-infiltrating B-cell plasma cells in the context of autoimmune lymphocytic thyroiditis. They are pathologically active rather than merely serological bystanders — mediating complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) against thyroid follicular cells, directly contributing to the progressive destruction of functional thyroid tissue that underlies the clinical hypothyroidism of established Hashimoto's thyroiditis. Anti-TPO titres tend to decline progressively over the natural history of Hashimoto's disease as antigen-bearing functional thyroid tissue is destroyed and the antigenic stimulus diminishes — a pattern that should not be misinterpreted as immunological remission.
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.
Why is the Anti-TPO test ordered?
- Hashimoto's thyroiditis diagnosis confirmation — patients with elevated TSH, low-normal Free T4, or a goitre where aetiological characterisation is clinically important; Anti-TPO positivity confirms the autoimmune mechanism, guides prognostic counselling, and informs levothyroxine decision-making in subclinical hypothyroidism
- Subclinical hypothyroidism treatment decision support — TSH in the 4.5–10 mIU/L range with normal Free T4 where Anti-TPO positivity shifts the clinical recommendation toward levothyroxine initiation; current ATA and AACE guidelines identify Anti-TPO positivity alongside elevated TSH as a factor supporting treatment rather than watchful waiting
- Euthyroid thyroid autoimmunity screening — patients with a strong family history of autoimmune thyroid disease, or those with other autoimmune conditions (type 1 diabetes, coeliac disease, vitiligo, Addison's disease, SLE, rheumatoid arthritis) where co-occurring thyroid autoimmunity prevalence is substantially higher than the general population
- Preconception thyroid autoimmunity assessment — women planning pregnancy where Anti-TPO positivity identifies those at substantially elevated risk of gestational hypothyroidism (requiring more aggressive TSH monitoring and lower treatment thresholds throughout pregnancy), miscarriage, preterm birth, and post-partum thyroiditis
- Pregnancy thyroid autoimmunity evaluation — pregnant women presenting with TSH above trimester-specific thresholds or with unexplained obstetric complications where Anti-TPO status informs the treatment urgency and post-partum monitoring plan
- Post-partum thyroiditis risk assessment — women presenting with new-onset thyroid dysfunction — either hyperthyroidism (destructive thyroiditis phase) or hypothyroidism — in the 3–12 months following delivery, where Anti-TPO positivity supports a post-partum autoimmune thyroiditis diagnosis and distinguishes it from Graves' disease hyperthyroidism
- Fertility and subfertility evaluation — women undergoing investigation for recurrent miscarriage, IVF failure, or unexplained subfertility where Anti-TPO status identifies autoimmune thyroid disease as a potentially treatable contributory factor, with emerging evidence supporting levothyroxine in Anti-TPO-positive euthyroid women undergoing ART
- Integrative and functional medicine thyroid assessment — Anti-TPO as the autoimmune marker component of a comprehensive thyroid evaluation, guiding decisions about gluten-elimination trials, selenium supplementation (which has modest evidence for Anti-TPO titre reduction), vitamin D optimisation, and immune modulation in patients whose conventional thyroid markers are normal but who have symptoms or family history suggesting autoimmune thyroid involvement
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — fasting is not required for Anti-TPO measurement. However, biotin supplementation must be withheld for a minimum of 48–72 hours before collection to prevent assay interference.
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Standard venipuncture into a serum separator tube (SST / gold top). No special preparation, fasting, or timing requirements apply beyond biotin washout. Document on the requisition form: (1) biotin supplementation — dose and whether withheld for ≥48–72 hours; (2) current immunosuppressive medications — corticosteroids, DMARDs, biologics — as these suppress Anti-TPO and may produce falsely negative results; (3) recent radioiodine therapy or thyroid surgery within the preceding 3 months — procedurally driven Anti-TPO elevation; (4) current thyroid hormone replacement — type and dose — and whether the morning dose was taken before the draw (thyroid medication does not affect Anti-TPO titre but should be documented for the complete clinical record); (5) pregnancy status and trimester; (6) known monoclonal antibody therapy — for heterophile antibody interference assessment. # REVIEWER FLAG: Confirm Access Medical Laboratories SST tube type, minimum serum volume, centrifugation and separation requirements, and specimen stability data for Anti-TPO before publication.
Specimen requirements
Minimum 0.5 mL serum. Centrifuge and separate serum from cells within 2 hours of collection. Stable at 2–8°C for up to 7 days; stable frozen at −20°C for extended storage. # REVIEWER FLAG: Confirm minimum volume and stability data with Access Medical Laboratories assay documentation.
What can affect Anti-TPO results?
High-dose biotin — commonly taken at 5–10 mg or more for hair and nail supplementation — interferes with streptavidin-biotin immunoassay architecture by saturating biotin-binding sites, competing with the biotinylated tracer antibody in competitive-format Anti-TPO assays and producing spuriously suppressed Anti-TPO results. This can reduce a clinically positive Anti-TPO titre to below the positivity threshold, masking thyroid autoimmunity. The direction of biotin interference in Anti-TPO assays is typically towards false-negative results — the opposite direction to TSH interference — making it critically important that biotin is withheld for a minimum of 48–72 hours before specimen collection whenever thyroid antibody testing is performed. # REVIEWER FLAG: Confirm Access Medical Laboratories Anti-TPO assay platform — biotin interference magnitude and direction depend on assay design; confirm whether the specific Access Medical Laboratories platform uses biotin-streptavidin capture chemistry.
Systemic immunosuppression — from corticosteroids, DMARDs, biologics, or chemotherapy — suppresses B-cell plasma cell activity and autoantibody production, reducing Anti-TPO titres below their uninhibited level. A patient on high-dose immunosuppressive therapy may test negative or low-positive for Anti-TPO despite underlying thyroid autoimmunity that would be seropositive in the absence of immunosuppression. Current immunosuppressive medications must be documented on the requisition form for accurate result interpretation.
Anti-TPO titres are generated in response to the continued antigenic stimulus of thyroid peroxidase released from damaged follicular cells. In end-stage Hashimoto's thyroiditis where extensive thyroid parenchymal destruction has eliminated most antigen-bearing tissue, the antigenic stimulus diminishes and Anti-TPO titres may fall — sometimes below the positivity threshold — despite the underlying autoimmune mechanism remaining intact. A previously positive Anti-TPO that has become negative in a long-standing hypothyroid patient should not be interpreted as autoimmune remission; it more likely reflects the natural titre decline of burned-out Hashimoto's disease.
Anti-TPO prevalence and titre increase with advancing age — reflecting the cumulative accrual of thyroid autoimmunity over the lifespan and the general increase in autoimmune reactivity with ageing. Low-titre Anti-TPO positivity (34–100 IU/mL) in elderly patients may represent age-related autoimmune background reactivity without clinically significant Hashimoto's thyroiditis progression risk, particularly when TSH is normal. Clinical context — TSH trajectory, family history, symptoms — must be integrated with the titre level before clinical decisions are made.
Radioiodine ablation causes acute thyroid follicular cell destruction and antigen release, transiently elevating Anti-TPO titres in the post-treatment period through increased antigen availability stimulating B-cell response. Anti-TPO measured within 6–12 weeks of radioiodine therapy reflects this procedurally driven elevation rather than the underlying pre-treatment autoimmune titre. Similarly, thyroid surgery with significant tissue manipulation may transiently alter Anti-TPO. Post-procedure timing must be documented when interpreting Anti-TPO results in patients with recent thyroid procedural history.
Human anti-animal antibodies — particularly human anti-mouse antibodies (HAMA) — generated through therapeutic monoclonal antibody treatment or occupational animal exposure can interfere with sandwich immunoassay capture antibodies, producing either spuriously elevated or spuriously suppressed Anti-TPO results inconsistent with clinical context. When Anti-TPO result appears discordant with clinical presentation, retesting after heterophile antibody blocking or on a different assay platform can identify this interference.
What do Anti-TPO results mean?
Elevated
A positive Anti-TPO result (above the Access Medical Laboratories positivity threshold) in a patient with elevated or progressively rising TSH confirms Hashimoto's thyroiditis as the aetiological mechanism of thyroid dysfunction and justifies levothyroxine initiation or strengthens the case for it in subclinical hypothyroidism. In a euthyroid patient, Anti-TPO positivity identifies thyroid autoimmunity and confers a 2–5% annual progression risk to overt hypothyroidism — TSH monitoring every 6–12 months is appropriate, with tighter monitoring in those with TSH above 2.5 mIU/L. In a patient with suppressed TSH, Anti-TPO positivity supports Graves' disease in the differential alongside TSI — approximately 75–85% of Graves' disease patients are Anti-TPO positive. In pregnancy or preconception, any Anti-TPO positivity above the threshold requires more frequent TSH monitoring, lower levothyroxine treatment thresholds, and post-partum thyroiditis surveillance. Very high titres (>500–1000 IU/mL) are not independently more clinically significant than lower positive titres for treatment decision-making — the binary positive/negative classification drives clinical decisions rather than titre level in most guidelines.
Normal
A borderline Anti-TPO (34–100 IU/mL) represents an indeterminate result — it indicates some degree of anti-TPO reactivity but does not meet the positivity threshold used in most outcome studies to define clinically significant thyroid autoimmunity. Borderline results should be interpreted in the clinical context: in a patient with rising TSH and a goitre, a borderline Anti-TPO supports an autoimmune mechanism and warrants monitoring. In a patient with entirely normal TSH and no clinical features, a borderline Anti-TPO result may reflect non-specific immunoreactivity and should be retested before clinical decisions are made. Borderline results in the preconception and pregnancy context warrant a conservative approach — treating as positive-equivalent for the purposes of monitoring intensity and levothyroxine threshold.
Reduced
A negative Anti-TPO (<34 IU/mL) in a patient with elevated TSH makes Hashimoto's thyroiditis less likely as the primary aetiology but does not exclude it — approximately 5–10% of Hashimoto's thyroiditis patients are Anti-TPO-negative, with Anti-TG being the sole detectable antibody. If autoimmune thyroid disease remains clinically suspected despite negative Anti-TPO, Anti-Thyroglobulin antibody (Anti-TG) testing is the appropriate next step. A negative Anti-TPO in a hypothyroid patient with negative Anti-TG should direct the investigation toward non-autoimmune causes of hypothyroidism — post-surgical, post-radioiodine, iodine excess (Wolff-Chaikoff effect), medication-induced (amiodarone, lithium, interferon), or central hypothyroidism from pituitary-hypothalamic pathology.
Note: The most important clinical caveat is that Anti-TPO titre level above the positivity threshold does not predict the rate of hypothyroidism progression or correlate with clinical symptom severity — a titre of 120 IU/mL and a titre of 1200 IU/mL carry the same positive binary classification and warrant the same monitoring response per guidelines. Serial titre monitoring — repeatedly measuring Anti-TPO over time to track changes — has limited clinical evidence as an actionable monitoring strategy in mainstream endocrinology, as titre fluctuations do not consistently predict progression rate or treatment response. A second caveat is that biotin interference with Anti-TPO assays produces false-negative results — in the opposite direction to TSH interference — making simultaneous biotin washout essential whenever thyroid antibody testing is performed alongside TSH. Finally, in pregnancy, Anti-TPO below the standard population positivity threshold may still carry obstetric significance — some obstetric guideline frameworks use lower pregnancy-specific Anti-TPO cut-offs for TSH monitoring intensification decisions that differ from the standard laboratory positivity threshold.
Frequently asked questions
A positive Anti-TPO result indicates the presence of autoantibodies against thyroid peroxidase — the enzyme central to thyroid hormone synthesis — confirming active thyroid autoimmunity. In a patient with elevated or borderline TSH, it confirms Hashimoto's thyroiditis as the aetiological mechanism. In a euthyroid patient, it identifies underlying autoimmunity and confers a 2–5% annual risk of progression to clinical hypothyroidism. In a patient with suppressed TSH, it supports Graves' disease as the hyperthyroid aetiology alongside TSI testing.
How to access the Anti-TPO test
The Anti-TPO reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: May 24, 2026
Last reviewed: May 24, 2026
- Garber JR, et al. Clinical practice guidelines for hypothyroidism in adults. Thyroid. (2012)
- Alexander EK, et al. 2017 Guidelines of the ATA for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. (2017)
- Stagnaro-Green A. Approach to the patient with postpartum thyroiditis. J Clin Endocrinol Metab. (2012)
- Duntas LH. The role of selenium in thyroid autoimmunity and cancer. Thyroid. (2006)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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