What Is Anti-TPO?
Thyroid peroxidase (TPO) is an enzyme involved in making thyroid hormone. Anti-TPO antibodies are produced by the immune system mistakenly attacking the thyroid gland — a hallmark of autoimmune thyroid disease.
Normal Anti-TPO Range
| Result | Range |
|---|---|
| Negative (normal) | Below 35 IU/mL |
| Positive (elevated) | 35 IU/mL and above |
What a Positive Result Means
Hashimoto's Thyroiditis
The most common cause of an underactive thyroid (hypothyroidism) in adults — anti-TPO is positive in roughly 90–95% of cases.
Graves' Disease
An autoimmune cause of overactive thyroid (hyperthyroidism); anti-TPO can also be positive, alongside TSI/TRAb antibodies.
When Is It Tested?
- Abnormal TSH, T3 or T4 results
- Goitre (enlarged thyroid)
- Family history of autoimmune thyroid disease
- Unexplained fatigue, weight changes or hair loss
- During pregnancy screening for thyroid risk
Important: A positive anti-TPO test does not always mean active thyroid disease — your doctor will interpret it alongside TSH, T3 and T4 results.