Thyroid Peroxidase Test

Picture of the thyroid gland.

Thyroid peroxidase (TPO) is an enzyme made in the thyroid gland that is important in the production of thyroid hormones. TPO is found in thyroid follicle cells, where it converts the thyroid hormone T4 to the thyroid hormone T3.

TPO also facilitates the binding of iodine to proteins within the thyroid gland, a process essential for synthesizing thyroid hormones T4 and T3.

The thyroid gland is located in the lower part of the neck, below the Adam's apple, wrapped around the trachea (windpipe).

In autoimmune thyroid diseases such as Hashimoto's thyroiditis and Graves' disease, the body produces antibodies against thyroid peroxidase (TPOAb). These antibodies can interfere with the normal activity of TPO, resulting in disrupted thyroid hormone production and thyroid dysfunction.

The thyroid peroxidase test is a test that measures the level of an antibody that is directed against thyroid peroxidase (TPO).

Autoantibodies to thyroid peroxidase (TPOAb) are produced within the body. The presence of TPOAb in the blood reflects a prior attack on the thyroid tissue by the body's immune system.

A TPO test can help evaluate the functionality of the thyroid gland and detect disorders related to the thyroid gland or thyroid hormones. Thyroid hormones play a vital role in various bodily processes such as metabolism, development, growth, and fertility.

The presence of TPOAb indicates the presence of autoimmune thyroid diseases, including:

• Hashimoto’s thyroiditis, which is a chronic inflammatory condition of the thyroid gland resulting in reduced thyroid activity, a condition called hypothyroidism.
• Graves’ disease, which results in hyperactivity of the thyroid gland, called hyperthyroidism.

Both hypothyroidism and hyperthyroidism can disrupt bodily functions and result in fatigue, weight changes, mood swings and affect fertility and pregnancy. It is advised to undergo TPO tests before or during pregnancy to detect thyroid diseases that can negatively affect both the mother and the unborn child.  A TPO test can also help monitor the progression of the condition.

Most people with chronic thyroiditis (Hashimoto’s disease or Graves’ disease) display a positive TPO test. The test is also positive in lesser numbers of people with other thyroid diseases.

Other autoimmune disorders that may cause a positive TPOAb test include:

• Sjogren's syndrome
• Lupus
• Rheumatoid arthritis
• Pernicious anemia

About 3% of people with elevated TPO antibodies show no evidence of disease. The chance of having a positive TPOAb test is greater in females and increases with age.

When thyroid peroxidase antibodies (TPOAb) are high, it typically indicates that your immune system is mistakenly attacking your thyroid gland, which can lead to thyroid issues. This is common in conditions such as Hashimoto's thyroiditis, in which the thyroid becomes underactive, and Graves' disease, in which it becomes overactive.

High TPOAb levels assist clinicians in diagnosing and monitoring the progression of various autoimmune thyroid diseases. It can also signify other autoimmune illnesses and possible thyroid difficulties during pregnancy. Elevated TPOAb levels indicate that more testing and monitoring may be required to successfully manage thyroid function.

The typical range for thyroid peroxidase antibodies (TPOAb) is less than 34 international units per milliliter (IU/mL). A TPOAb level below this threshold is considered normal, indicating no significant autoimmune activity against your thyroid.

What if thyroid peroxidase is high?

If the level is higher than 34 IU/mL, it suggests the existence of these antibodies, which may indicate an autoimmune thyroid illness such as Hashimoto's thyroiditis or Graves' disease. Elevated TPOAb levels indicate that your immune system is attacking your thyroid gland, which may affect its function.

What is a good level for thyroid peroxidase antibody?

Having TPOAb levels within the normal range, typically less than 34 IU/mL, is considered optimal and is often not associated with autoimmune thyroid disease.

What does it mean if your thyroid peroxidase is low?

Low or undetectable TPOAb levels are typically not a cause for concern, indicating minimal autoimmune activity against the thyroid gland and suggesting a lower risk of autoimmune thyroid disorders.

A positive TPO result indicates that your blood test has detected high amounts of thyroid peroxidase antibodies. This means that your immune system is attacking your thyroid gland, which is said to be an autoimmune thyroid condition. A positive TPO result is commonly associated with Hashimoto's thyroiditis (which can lead to an underactive thyroid) and Graves' disease (which can cause an overactive thyroid).

A positive result helps doctors diagnose these conditions and determine the appropriate treatment plan for managing your thyroid health.

This test goes by a confusing number of names, including:

• Thyroid peroxidase antibody (TPOAb) test
• Thyroid peroxidase autoantibody test
• Antimicrosomal antibody test
• Antithyroid microsomal antibody test