Anti-TPO, or anti-thyroid peroxidase antibodies, are proteins produced by the immune system and found in the blood, often indicating thyroid health. Testing for Anti-TPO antibodies is a common diagnostic tool to investigate thyroid concerns, helping healthcare providers understand if an immune response is targeting the thyroid.
Understanding Thyroid Peroxidase and Its Antibodies
Thyroid peroxidase (TPO) is an enzyme found in the thyroid gland, a butterfly-shaped organ at the base of the neck. This enzyme plays a central role in producing thyroid hormones (thyroxine, T4, and triiodothyronine, T3) by facilitating iodine addition to thyroglobulin. Thyroid hormones regulate metabolism, growth, and development.
Anti-TPO antibodies are autoantibodies, mistakenly produced by the immune system against the body’s own tissues. The immune system incorrectly identifies TPO as a threat, generating antibodies to attack it. This autoimmune response can inflame and damage the thyroid gland, indicating the immune system is targeting its hormone-producing machinery.
Why the Anti-TPO Test is Conducted
A healthcare provider might order an Anti-TPO test when an autoimmune thyroid condition is suspected, often as part of a broader thyroid function evaluation. Symptoms like persistent fatigue, unexplained weight changes, or difficulty tolerating cold temperatures, hair loss, dry skin, constipation, or muscle weakness may prompt testing.
The test is useful if initial thyroid hormone tests, such as TSH, indicate a potential thyroid imbalance, helping determine if an autoimmune process contributes to dysfunction. A family history of thyroid issues or an enlarged thyroid gland (goiter) may also prompt testing, even if hormone levels are currently normal.
Interpreting Anti-TPO Test Results
Interpreting Anti-TPO test results involves understanding what the detected levels of antibodies signify. A “normal” or “negative” result typically means that Anti-TPO antibodies were not detected or are below a certain laboratory-specific threshold. While reference ranges can vary between laboratories, a common normal range for Anti-TPO antibodies is often considered to be less than 9 IU/mL or less than 35 IU/mL. A negative result suggests there is no significant autoimmune activity against the thyroid.
Conversely, “elevated” or “positive” Anti-TPO levels indicate the presence of these antibodies, suggesting an autoimmune process is occurring. Elevated Anti-TPO is strongly linked to autoimmune thyroid diseases. It is found in approximately 90% to 95% of individuals with Hashimoto’s thyroiditis, a condition that often leads to an underactive thyroid (hypothyroidism). High levels can also be present in about 65% to 80% of individuals with Graves’ disease, which typically causes an overactive thyroid (hyperthyroidism).
The presence of these antibodies means the immune system is targeting the thyroid, even if current thyroid hormone levels appear normal. This can indicate an increased risk of developing thyroid dysfunction in the future, as approximately 10-15% of people without overt thyroid disease may also have elevated Anti-TPO antibodies.
Next Steps After Testing
Receiving Anti-TPO test results, particularly if they are elevated, requires a discussion with a healthcare provider to understand the implications for individual health. The presence of these antibodies indicates an underlying autoimmune process, which may or may not be causing immediate thyroid dysfunction. Further diagnostic tests are typically ordered to assess the overall function and structure of the thyroid gland.
These additional tests often include measurements of Thyroid Stimulating Hormone (TSH), free T4, and sometimes free T3 to get a complete picture of thyroid hormone levels. A thyroid ultrasound may also be recommended to visualize the thyroid gland and check for any structural changes. Management strategies will depend on the comprehensive clinical picture, including symptoms and other thyroid hormone levels, rather than just the antibody levels alone. Regular monitoring of thyroid function tests is common, especially if antibody levels are elevated but hormone levels are currently normal, to track any progression.