Anti-TPO are antibodies against thyroid peroxidase, the key enzyme with which the thyroid gland synthesizes hormones. Their appearance means the immune system is mistakenly attacking its own gland.
This is the main marker of autoimmune thyroid disease, primarily Hashimoto's thyroiditis. Anti-TPO does not show how the gland works right now (that is what TSH and free T4 are for), but it explains the cause of the disorder and the future risk.
Reference ranges
This is not a "hormone level" but an antibody titer — assessed against the lab's threshold:
| Negative (normal) | usually < 34 IU/mL |
| Positive | > 34 IU/mL (threshold depends on the lab) |
| Hashimoto's thyroiditis | anti-TPO positive in ~90% of patients |
| Carriage | antibodies "+" with normal TSH and free T4 |
What an elevated level means
A positive (elevated) anti-TPO means an autoimmune process in the thyroid gland.
Most often this is Hashimoto's thyroiditis — the most common cause of hypothyroidism: for years the antibodies damage the gland, and it gradually loses the ability to produce hormones. Anti-TPO can also be positive in Graves' disease and postpartum thyroiditis.
What it means in practice: even with a normal TSH, positive anti-TPO is an increased risk of developing hypothyroidism in the future. Such people are advised to monitor TSH periodically.
What a low level means
A negative result (antibodies not detected or below the threshold) is normal: there is no autoimmune damage by this marker.
Two important caveats. A small percentage of healthy people (especially women) have low-positive anti-TPO without disease — this is carriage, not a diagnosis. And conversely, with already-confirmed Hashimoto's there is no need to re-check the titer: it does not reflect disease severity, and treatment is prescribed by TSH and free T4, not by the antibody level.
How the test is done
Anti-TPO is drawn from a vein; no special preparation is required; biotin is stopped 2–3 days before. The test usually needs to be done once — to determine whether the disorder is autoimmune in nature.
Anti-TPO is ordered together with TSH and free T4: the hormones show gland function now, and the antibodies show the cause and prognosis.
What affects the value
- Biotin (vitamin B7) in large doses distorts the result — stop it 2–3 days before.
- Lab method and threshold — "positive/negative" depends on the test system.
- Other autoimmune diseases (type 1 diabetes, vitiligo, celiac disease) — more often accompanied by anti-TPO.
- Pregnancy and the postpartum period — anti-TPO raises the risk of postpartum thyroiditis.
- Sex and age — antibody carriage is more common in women and increases with age.
Assess thyroid function
Anti-TPO shows the cause, while these calculators show how the gland works now:
Frequently asked questions
Anti-TPO is positive but hormones are normal — is that a disease?
Not yet. This is antibody carriage: an autoimmune process is present, but the gland is still coping. However, the risk of developing hypothyroidism over time is increased, so monitoring TSH every 6–12 months is recommended.
Do I need to lower anti-TPO antibodies?
There is no need — and it is usually impossible — to specifically "lower the titer": it is not the antibodies that are treated but gland function. If Hashimoto's has caused hypothyroidism, levothyroxine is prescribed by TSH and free T4. The anti-TPO titer itself does not affect treatment strategy.
Do I need to re-test anti-TPO over time?
Usually no. It is enough to establish once that the process is autoimmune. The titer does not reflect disease severity and changes little with treatment, so TSH and free T4 are used for monitoring, not repeat antibodies.