Also seen on reports as: TPO Ab, Anti-TPO Ab, Thyroid Peroxidase Antibodies, Anti-microsomal Antibody (older term)
What is Anti-TPO?
Anti-TPO antibodies are proteins the immune system can make against thyroid peroxidase — an enzyme that the thyroid gland uses to produce its hormones (T3 and T4). When the immune system mistakenly targets thyroid peroxidase, it can interfere with the thyroid's normal hormone production.
A high Anti-TPO level is the laboratory marker of autoimmune thyroid disease — most commonly Hashimoto's thyroiditis (the leading cause of hypothyroidism worldwide and in India) and, less often, Graves' disease (a cause of hyperthyroidism).
The test is run on a small blood sample drawn from a vein. NABL-accredited Indian labs typically report Anti-TPO in IU/mL. Different labs use different cut-offs for what counts as 'positive', so always interpret the value against the range printed on the same report.
Why doctors order it
Anti-TPO is most often ordered after an abnormal TSH or Free T4, to find out whether the underlying problem is autoimmune. The combination of a raised TSH and a positive Anti-TPO strongly points to Hashimoto's thyroiditis as the cause of hypothyroidism.
Doctors also order Anti-TPO in subclinical hypothyroidism (mildly raised TSH with normal Free T4) to help decide whether to treat or just monitor. A positive Anti-TPO in subclinical hypothyroidism makes progression to overt hypothyroidism more likely, which often shifts the decision toward earlier treatment.
Anti-TPO is part of the antenatal panel in many Indian obstetric practices. A positive Anti-TPO during pregnancy raises the risk of post-partum thyroiditis (a thyroid disturbance that develops in the first year after delivery) and changes follow-up planning.
What the result means
A positive Anti-TPO means the immune system is making antibodies against the thyroid. It does not by itself mean the thyroid is currently failing — many people have detectable Anti-TPO with completely normal thyroid function, particularly women. Whether and how much it matters depends on the TSH and Free T4.
A high Anti-TPO with a high TSH and low Free T4 is the typical Hashimoto's pattern. A high Anti-TPO with a normal TSH/Free T4 is sometimes called 'euthyroid Hashimoto's' — the antibodies are there, but thyroid function is still in range. Such people are at higher risk of developing hypothyroidism over time and are usually monitored periodically.
Anti-TPO is also positive in many people with Graves' disease (a cause of hyperthyroidism), though TSH receptor antibodies (TRAb) and TSI are more specific for that diagnosis. Anti-TPO is not usually used to monitor treatment — once positive, the level can fluctuate over years without necessarily reflecting clinical change.
Reference range
Reference ranges vary between Indian labs — interpret against the cut-off on the same report
- Negative / normal
- Below the lab's cut-off (often under 35 IU/mL, but lab-dependent)
- Positive
- Above the lab's cut-off — autoimmune thyroid disease likely
- Strongly positive
- Several times the cut-off — high specificity for autoimmune thyroid disease
- Positive with normal TSH/T4
- Risk factor for developing thyroid disease over time; periodic monitoring usual
Anti-TPO results vary substantially between Indian labs because antibody assays are not fully standardised across manufacturers. Two readings from different labs cannot be directly compared. Repeat testing usually offers little benefit once positivity is established.
Common factors that affect the result
Anti-TPO levels can be detectable in people without any thyroid problem — particularly women, and particularly with age. A positive result in someone with normal TSH and Free T4 is not the same as a diagnosis of thyroid disease. It is a risk marker, not a verdict.
Levels can fluctuate without obvious clinical change, and they do not move predictably with treatment. So Anti-TPO is generally not used to monitor someone already known to have autoimmune thyroid disease — TSH and Free T4 are used for that.
Biotin supplements (common in hair and nail products in India) interfere with many antibody assays and can produce misleading Anti-TPO results. Most guidelines recommend stopping biotin 48–72 hours before any thyroid antibody testing.
When to talk to your doctor
- Your TSH is raised and Anti-TPO is positive — this is the typical Hashimoto's pattern and usually needs follow-up planning, even if Free T4 is still normal.
- You are pregnant or trying to conceive and Anti-TPO is positive — this changes pregnancy thyroid monitoring and may affect TSH targets.
- You have post-partum thyroid symptoms (palpitations, weight changes, severe tiredness) and a positive Anti-TPO from pregnancy testing — post-partum thyroiditis is treatable but needs identification.
- Anti-TPO is positive but TSH and Free T4 are normal — periodic monitoring of TSH is usually planned, with no immediate treatment needed.
- You have a family history of thyroid disease and your Anti-TPO has come back positive — your doctor may suggest a baseline TSH and a follow-up schedule.
Related tests
Sources
- Indian Thyroid Society Guidelines for Management of Hypothyroidism
- ATA Clinical Practice Guidelines for Hypothyroidism in Adults — American Thyroid Association
- FOGSI Guidelines on Thyroid in Pregnancy — Federation of Obstetric and Gynaecological Societies of India
- WHO Thyroid Disorders Guidance — World Health Organization
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