What this test measures
Thyroid Peroxidase (TPO) is the enzyme that incorporates iodine into thyroid hormone synthesis. TPO antibody is an autoantibody directed against this enzyme — the most sensitive single immunological marker of autoimmune thyroid disease.
Levels can range from a few units in healthy people to several thousand IU/mL in active Hashimoto's. The assay is quantitative and a single positive result is usually all that is needed for diagnostic purposes — antibody levels do not need repeating once positive.
Why it matters
Hashimoto's thyroiditis is the commonest cause of hypothyroidism in India — about 10% of adults are affected, overwhelmingly women. TPO antibody is positive in over 90% of patients with Hashimoto's and 50–70% of Graves' disease. The American Thyroid Association, Indian Thyroid Society and most international bodies recommend TPO testing in any woman with subclinical hypothyroidism planning pregnancy, in unexplained subfertility and recurrent miscarriage, and to confirm an autoimmune cause before committing to lifelong levothyroxine.
A positive TPO with a normal TSH carries a 5% per year risk of progression to overt hypothyroidism, and a higher risk of postpartum thyroiditis in women — so antibody-positive patients deserve annual TSH follow-up.
How to prepare
No fasting required. Stop biotin / cosmetic supplements 48–72 hours before. Continue thyroid hormone replacement. Tell the lab if you have recently had 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 |
|---|---|---|---|
| TPO Antibody (IU/mL)[1][2] | < 35 IU/mL (assay-dependent) | Negative TPO largely rules out autoimmune thyroid disease but does not exclude it — a small minority of patients are TPO-negative but anti-Tg positive. | Positive TPO is highly suggestive of autoimmune thyroid disease. Strong positivity (> 500) is essentially diagnostic of Hashimoto's. Lower positivity is also seen in Graves', postpartum thyroiditis and as an incidental finding in otherwise healthy women. |
TPO antibody — interpretation
| TPO | TSH | Likely picture | Action |
|---|---|---|---|
| Strongly positive | Raised | Overt Hashimoto's hypothyroidism | Start levothyroxine |
| Positive | Mildly raised | Subclinical Hashimoto's | Treat if pregnant / symptomatic; else monitor |
| Positive | Normal | Euthyroid autoimmunity | Annual TSH; closer monitoring in pregnancy |
| Positive | Suppressed | Graves' or hashitoxicosis | Add TRAb; endocrine referral |
| Negative | Raised | Non-autoimmune hypothyroidism | Look for iodine status, drugs, congenital cause |
Frequently asked questions
My TPO is high but TSH is normal — do I need treatment?
Not routinely. A positive TPO with a normal TSH means autoimmune tendency but a functioning gland. About 5% per year progress to overt hypothyroidism — annual TSH monitoring is recommended. Treatment threshold is lower in pregnancy.
Will TPO antibodies ever come down?
In some patients antibody titres fall over years on adequate levothyroxine; in many they remain detectable lifelong. The treatment target is normal TSH, not antibody negativity.
I am planning pregnancy — what does a positive TPO mean?
Positive TPO in pregnancy is linked with 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.
Why are women so much more affected than men?
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 likely contributors.
My child has Hashimoto's — will mine reach the same level?
Antibody levels do not correlate with disease severity — once positive, the absolute number does not predict who will progress how fast. TSH trends matter more.
Can iodine excess trigger Hashimoto's?
Excess iodine intake (very high-dose kelp / iodine supplements) can unmask or worsen autoimmune thyroid disease in genetically predisposed people. Routine dietary iodine in salt is not a problem.
Should anti-Tg also be tested?
Anti-Tg adds a small percentage of Hashimoto's patients who are TPO-negative. The ATAB panel (TPO + Tg) is the typical Indian lab bundle.
Related Hormones / Endocrine tests
Tests commonly ordered alongside TPO (Thyroid Peroxidase) Antibody, 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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