What is TPO?

Thyroid peroxidase (TPO) is an enzyme normally found in the thyroid gland. Thyroid peroxidase helps to make thyroid hormones (T3, T4 and TSH).

Where is the thyroid gland?

The thyroid is a small bow-shaped gland that sits in the front of the neck, just below the Adam's apple.

The thyroid gland produces three hormones:

- Triiodothyronine, also known as T3.

- Tetraiodothyronine, also called thyroxine or T4.

- Calcitonin.

What are the functions of TPO?

TPO plays an important role in the production of thyroid hormones. Thyroid peroxidase assists the chemical reaction that adds iodine to a protein called thyroglobulin, a critical step in generating thyroid hormones. Thyroid hormones play an important role in regulating growth, brain development, and the rate of chemical reactions in the body (metabolism).

What are TPO antibodies?

Thyroid peroxidase (TPO) antibodies are a type of thyroid antibody. The body’s immune system makes antibodies in response to non-self proteins. These non-self proteins are called antigens. Making antibodies is important to protect us from diseases. However sometimes the immune system identifies our own proteins as non-self, making autoantibodies.

The immune system then attacks our own body proteins in error. This is known as an autoimmune disease. Thyroid antibodies develop when the immune system identifies our own thyroid cells and tissues as non-self. This can cause inflammation and affect thyroid function. 

Why test for TPO antibodies?

- A TPO test detects antibodies against TPO in the blood. If you've been diagnosed with thyroid disease, your doctor may suggest a TPO antibody test and other thyroid tests to help find the cause.

- The presence of TPO antibodies in your blood suggests that the cause of thyroid disease is an autoimmune disorder, such as Hashimoto's disease or Graves' disease. In autoimmune disorders, your immune system makes antibodies that mistakenly attack normal tissue. Antibodies that attack the thyroid gland cause swelling, rarely tenderness and reduced function of the thyroid.

- Some people with TPO antibodies may not have thyroid disease. But the presence of TPO antibodies may increase the risk of future thyroid disorders.

What are the two main thyroid autoimmune diseases?

The two main autoimmune thyroid diseases are Grave’s disease and Hashimoto’s thyroiditis.

Graves’ disease (=increased hormone productions) is an autoimmune disorder that can cause hyperthyroidism, or overactive thyroid. With Graves’ disease, your immune system attacks your thyroid gland, causing it to make more thyroid hormones than your body needs. As a result, many of your body’s functions speed up.

Hashimoto’s thyroiditis (=impaired hormone production), also known as chronic lymphocytic thyroiditis, is the most common cause of hypothyroidism in the United States. It is an autoimmune disorder involving chronic inflammation of the thyroid. This condition tends to run in families. Over time, the ability of the thyroid gland to produce thyroid hormones often becomes impaired and leads to a gradual decline in function and eventually an underactive thyroid (Hypothyroidism). Hashimoto’s thyroiditis occurs most commonly in middle aged women, but can be seen at any age, and can also affect men and children.

What is the thyroid peroxidase antibody normal range?

Normal TPO antibodies range is less than 34 international units per millimetre (IU/ml).  Thyroid peroxidase antibodies are considered high if above this. Note that the normal range used can differ in different laboratories. It is not possible for the thyroid peroxidase antibodies to be too low.

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Normal range for Thyroid Peroxidase Antibodies:

Quest Diagnostics: <9 IU/mL

LabCorp: adults: <34 IU/mL

Other age ranges by LabCorp: 

What is the difference between thyroid peroxidase antibodies and thyroglobulin antibodies?

Thyroid peroxidase and thyroglobulin antibodies are two different types of thyroid antibodies. The body needs thyroglobulin and thyroid peroxidase to make thyroid hormones. Both types of thyroid antibodies are often raised in autoimmune thyroid disorders. Raised thyroid peroxidase antibodies are found in a higher percentage of people with autoimmune thyroid diseases. 

References:

Frohlich E, Wahl R. (2017).Thyroid Autoimmunity: Role of Anti-thyroid Antibodies in Thyroid and Extra-Thyroidal Diseases. Front Immunol.9;8:521. PMID: 28536577; PMCID: PMC5422478. 

Godlewska M et al. (2019). Thyroid Peroxidase Revisited–What's New?. Hormone and Metabolic Research, 51(12), 765-769.

Rrupulli A et al. (2019). Significance of testing anti-thyroid peroxidase in euthyroid patients. In Endocrine Abstracts (Vol. 63). Bioscientifica.

Xiao Y et al. (2019). Positive thyroid antibodies and risk of thyroid cancer: A systematic review and meta-analysis. Molecular and clinical oncology, 11(3), 234–242.

Rayman, M. (2019). Multiple nutritional factors and thyroid disease, with particular reference to autoimmune thyroid disease. Proceedings of the Nutrition Society, 78(1), 34-44.

Chahardoli R et al. (2019). [Can supplementation with vitamin D modify thyroid autoantibodies (Anti-TPO Ab, Anti-Tg Ab) and thyroid profile (T3, T4, TSH) in Hashimoto’s thyroiditis? A double blind, Randomized clinical trial. Hormone and Metabolic Research, 51(05), 296-301](https://doi.org/ 10.1055/a-0856-1044).

Zhang W et al. (2021) Immunomodulatory function of vitamin D and its role in autoimmune thyroid disease. Frontiers in immunology, 12, 574967. 

Thyroid function tests. American Thyroid Association. https://www.thyroid.org/thyroid-function-tests/.

Overview of thyroid function. Merck Manual Professional Version. https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/thyroid-disorders/overview-of-thyroid-function?query=overview thyroid function.

Postpartum thyroiditis. American Thyroid Association. http://www.thyroid.org/postpartum-thyroiditis/

Jameson JL, et al., eds. Thyroid gland physiology and testing. In: Harrison's Principles of Internal Medicine. 20th ed. The McGraw-Hill Companies; 2018. https://accessmedicine.mhmedical.com

Laposata M. The endocrine system. In: Laposata's Laboratory Medicine: Diagnosis of Disease in the Clinical Laboratory. 3rd ed. The McGraw-Hill Companies; 2019. https://accessmedicine.mhmedical.com