What this test measures
Anti-Thyroid Peroxidase antibody (anti-TPO) is an autoantibody directed against thyroid peroxidase — the enzyme that incorporates iodine into thyroid hormone synthesis. The presence of anti-TPO is the immunological fingerprint of autoimmune thyroid disease.
The test is run as a quantitative immunoassay reported in IU/mL. Levels can range from a few units in healthy people to several thousand in active Hashimoto's thyroiditis.
Why it matters
Hashimoto's thyroiditis is by far the commonest cause of hypothyroidism in India. Population studies put the prevalence of hypothyroidism at around 10%, of which the majority is autoimmune. Anti-TPO is positive in over 90% of patients with Hashimoto's and in 50–70% with Graves' disease.
A positive anti-TPO matters even with a normal TSH: about 5% of such people develop overt hypothyroidism each year, and they are at higher risk of pregnancy complications (miscarriage, postpartum thyroiditis). Indian guidelines and the American Thyroid Association recommend anti-TPO testing in any woman with subclinical hypothyroidism planning pregnancy, in unexplained subfertility and recurrent miscarriage, and to confirm an autoimmune cause before committing a patient to lifelong levothyroxine.
How to prepare
No fasting required. Stop biotin / hair-skin-nail supplements for 48–72 hours before — high-dose biotin interferes with most thyroid immunoassays. Continue thyroid hormone replacement as usual. Tell the lab if you have had recent iodine-contrast imaging.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Anti-TPO (IU/mL)[1][2] | < 35 IU/mL (assay-dependent; typical 9–35) | A negative anti-TPO largely rules out autoimmune thyroid disease, but does not exclude it completely — a small minority of patients are TPO-negative but anti-Tg positive. | Positive anti-TPO is highly suggestive of autoimmune thyroid disease. Levels above 500 IU/mL almost always indicate Hashimoto's. Moderate positivity is also seen in Graves' disease, postpartum thyroiditis and as an incidental finding in healthy women. |
Anti-TPO patterns and what they mean
| Anti-TPO | TSH | Likely picture | Action |
|---|---|---|---|
| Strongly positive | Raised | Overt Hashimoto's hypothyroidism | Start levothyroxine; recheck TSH 6–8 weeks |
| Strongly positive | Mildly raised | Subclinical Hashimoto's | Treat if pregnant / symptomatic; else monitor |
| Positive | Normal | Euthyroid autoimmunity | Annual TSH; pregnancy needs closer monitoring |
| Positive | Suppressed | Graves' disease or hashitoxicosis | Add TRAb; endocrine referral |
| Negative | Raised | Non-autoimmune hypothyroidism | Consider iodine status, drugs, congenital cause |
Frequently asked questions
My anti-TPO is high but TSH is normal — do I need treatment?
Not routinely. A positive anti-TPO with a normal TSH means you have autoimmune thyroid tendency but the gland is still functioning. About 1 in 20 people in this group develop overt hypothyroidism each year, so annual TSH monitoring is recommended. If you are pregnant or planning pregnancy, the threshold for treatment is lower.
Will anti-TPO ever become negative?
In a minority of patients antibodies fall over years on adequate levothyroxine; in most, they remain detectable lifelong. The goal of treatment is a normal TSH, not antibody negativity.
Does a high antibody level mean my disease will be worse?
Higher titres correlate weakly with faster progression to overt hypothyroidism but do not change the eventual treatment. Once you are on the right dose of levothyroxine, the actual antibody number stops mattering clinically.
I am pregnant — what does a positive anti-TPO change?
Pregnancy with positive anti-TPO carries a higher risk of miscarriage, preterm birth and postpartum thyroiditis. Many obstetricians treat subclinical hypothyroidism more aggressively in antibody-positive women and recommend a TSH check at 6 weeks and 6 months postpartum.
Can I prevent Hashimoto's if I have a family history?
No proven preventive strategy exists. Avoid iodine excess (very high-dose kelp/iodine supplements), maintain healthy vitamin D and selenium status, and check TSH every 1–2 years from age 30 if there is a strong family history.
Why are women affected so much more?
Autoimmune thyroid disease is 5–8× more common in women, with peaks at puberty, after pregnancy and at perimenopause. Oestrogen and X-chromosome immune-related genes are thought to be major contributors.
Will my child develop Hashimoto's?
There is a real familial tendency — first-degree relatives of Hashimoto's patients have a 4–5× higher lifetime risk. But most children of antibody-positive mothers will never develop overt disease. Routine paediatric screening is not recommended in the absence of symptoms.
Related Hormones / Endocrine tests
Tests commonly ordered alongside ANTI TPO, or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Hypothyroidism Guideline · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Thyroid Antibodies Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Hashimoto Thyroiditis · accessed 2026-05-30T00:00:00.000Z
- Indian Thyroid Society — Consensus · accessed 2026-05-30T00:00:00.000Z
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