Anti-TPO are autoantibodies to thyroid peroxidase, a key enzyme in the synthesis of thyroid hormones. It is the main laboratory marker of autoimmune damage to the gland.
Elevated anti-TPO is a characteristic sign of autoimmune thyroiditis (Hashimoto's). The same test is also designated as TPO antibodies.
Reference ranges
The reference range depends on the assay; the result is interpreted as "negative" or "positive":
| Negative (normal) | usually < 34–35 IU/mL |
| Positive | above the laboratory threshold |
What an elevated level means
Elevated anti-TPO indicates an autoimmune process. Most often this is Hashimoto's thyroiditis; the antibodies are also often positive in Graves' disease. A high titer with normal gland function means an increased risk of developing hypothyroidism in the future, but does not itself require treatment.
What a low level means
A negative (normal) result makes autoimmune damage to the gland unlikely. That said, other causes of impaired thyroid function are not ruled out, so anti-TPO is assessed together with TSH and free T4.
How the test is done
Blood is drawn from a vein; no special preparation is needed. The absolute value of the titer correlates poorly with the severity of the disease, so repeatedly "tracking the number" over time is usually not necessary — a single confirmation of the autoimmune nature is enough.
What affects the value
- A small rise occurs in some healthy people, especially in women and with age.
- The antibodies are also found in other autoimmune diseases (type 1 diabetes, celiac disease).
- During pregnancy, positive anti-TPO raises the risk of postpartum thyroiditis.
Calculate using this marker
Calculators where Anti-TPO (thyroid peroxidase antibodies) is used directly:
Frequently asked questions
Anti-TPO is positive but TSH is normal — what should I do?
No special treatment is needed. It only means an increased risk of hypothyroidism, so periodic monitoring of TSH is recommended (usually once a year), along with attention to symptoms.
Can anti-TPO be lowered?
There is no reliable way to deliberately lower the titer, and there is no clinical need to: what matters is not the antibody number but gland function (TSH and free T4). That is what is corrected when necessary.