Thyroid peroxidase antibodies (anti-TPO) are produced by the immune system and attack the thyroid gland, which can lead to changes in thyroid hormone levels.
The normal range for thyroid peroxidase antibodies can vary depending on the lab running the test, but levels above 35.0 IU/mL are generally considered elevated. High levels may indicate thyroid conditions such as Hashimoto’s thyroiditis or Graves’ disease.
However, the anti-TPO levels can increase in different situations. For this reason, results should be interpreted by an endocrinologist together with other thyroid tests, such as TSH, T3, and T4 levels.
Reference values
The normal reference value for thyroid peroxidase antibodies is typically less than 35.0 IU/mL.
This value may vary depending on the lab running the test, as different testing methods can influence what is considered a normal range. Therefore, it’s important to check the reference range provided on the lab report when interpreting results.
Anti-TPO levels above 35.0 IU/mL may suggest a thyroid disorder, so the results should always be evaluated by a healthcare provider alongside other thyroid tests.
Causes of high anti-TPO levels
Thyroid peroxidase antibodies are considered high when their concentration in the blood is above 35.0 IU/mL. This may indicate:
1. Hashimoto’s thyroiditis
Hashimoto’s thyroiditis is an autoimmune condition in which the immune system attacks the thyroid gland, disrupting thyroid hormone production. This can lead to hypothyroidism, with symptoms such as fatigue, weight gain, muscle pain, and brittle hair and nails.
Hashimoto’s thyroiditis is one of the main causes of elevated thyroid peroxidase antibodies, although further tests are needed to confirm the diagnosis.
What to do: Treatment is managed by an endocrinologist or primary care provider and typically involves levothyroxine to correct the hypothyroidism caused by Hashimoto’s thyroiditis.
2. Graves’ disease
Graves’ disease is another common cause of high thyroid peroxidase antibody levels.
In Graves’ disease, thyroid-stimulating antibodies (TSH receptor antibodies) cause the thyroid to produce too much hormone. This often results in high anti-TPO levels as a sign of autoimmune activity.
Symptoms of hyperthyroidism may include headaches, bulging eyes, weight loss, sweating, muscle weakness, and swelling in the neck.
What to do: Treatment depends on the severity of the disease and may include antithyroid medications (such as methimazole or propylthiouracil), radioactive iodine therapy, or thyroid surgery, as recommended by an endocrinologist.
3. Pregnancy
During pregnancy, hormonal changes can affect the thyroid gland, sometimes leading to an increase in thyroid peroxidase antibodies in the blood.
However, having elevated anti-TPO levels while pregnant does not necessarily mean there is a thyroid disorder.
What to do: The anti-TPO test is often performed early in pregnancy so the doctor can monitor levels throughout gestation and assess the risk of developing postpartum thyroiditis.
4. Subclinical hypothyroidism
Subclinical hypothyroidism occurs when the thyroid gland is underactive but does not cause noticeable symptoms. It is typically identified through blood tests showing normal T4 levels and high TSH.
Although the anti-TPO test is not commonly used to diagnose subclinical hypothyroidism, a healthcare provider may order it to evaluate disease progression and monitor response to treatment.
This test can help determine whether a decrease in thyroid peroxidase antibody levels corresponds to normalization of TSH levels, since these antibodies act directly on the enzyme involved in thyroid hormone production.
What to do: Treatment is prescribed by an endocrinologist or primary care provider and may involve levothyroxine, even in the absence of symptoms, to help lower cardiovascular risk.
5. Family history
People with a family history of autoimmune thyroid disorders may also have elevated thyroid peroxidase antibodies, even if they do not have thyroid disease themselves.
For this reason, anti-TPO results should always be interpreted together with other thyroid test results ordered by a healthcare provider.
What to do: Regular monitoring and periodic thyroid testing are recommended to detect any thyroid changes early and start treatment if necessary.