Thyroid peroxidase (TPO) antibodies are a key marker for assessing thyroid health. They offer insight into how the immune system interacts with the thyroid gland, helping evaluate certain thyroid conditions.
What Are TPO Antibodies?
TPO antibodies are proteins produced by the immune system that mistakenly target an enzyme called thyroid peroxidase (TPO). Thyroid peroxidase is an enzyme found in the thyroid gland that helps in the production of thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4). It plays a role in adding iodine to a protein called thyroglobulin, which is a necessary step in hormone synthesis.
When the immune system identifies TPO as a foreign substance, it creates autoantibodies to attack it. The presence of these autoantibodies indicates an autoimmune process, where the body’s own immune system targets its healthy cells and tissues. Measuring these antibodies helps determine if thyroid dysfunction is due to an autoimmune condition.
Understanding TPO Antibody Test Results
TPO antibody levels are measured through a blood test. Results are compared against a “reference range” or “normal range,” which can vary between laboratories. A negative result means the antibody levels are within that lab’s normal range.
When TPO antibody levels are elevated, it suggests an autoimmune process targeting the thyroid. However, an elevated level alone does not always mean active thyroid disease, as some individuals with elevated TPO antibodies may still have normal thyroid function.
How TPO Antibodies Indicate Hashimoto’s
Elevated TPO antibodies are a strong indicator of Hashimoto’s thyroiditis, an autoimmune disease where the immune system gradually destroys the thyroid gland. While there isn’t one universal threshold for diagnosis, TPO antibody levels above 35 IU/mL are generally considered elevated and may suggest autoimmune thyroid disease. Some studies consider levels above 200 units/ml or even 500 units/ml to be high. Mayo Clinic Laboratories, for instance, considers values above 9.0 IU/mL to be associated with autoimmune thyroiditis.
Higher TPO antibody levels can correlate with a more active autoimmune process. TPO antibodies are found in about 80-95% of individuals with Hashimoto’s thyroiditis. However, some people with Hashimoto’s may not have elevated TPO antibodies, and conversely, some individuals can have elevated TPO antibodies without currently having full-blown Hashimoto’s.
Comprehensive Hashimoto’s Diagnosis
TPO antibody levels are one piece of information used in diagnosing Hashimoto’s thyroiditis. Healthcare providers also consider other diagnostic tests, such as Thyroid Stimulating Hormone (TSH) levels and Free Thyroxine (Free T4) levels. TSH is produced by the pituitary gland, and high TSH levels can indicate that the thyroid gland is not producing enough hormones, a condition known as hypothyroidism. Low levels of Free T4, the main hormone produced by the thyroid, further confirm hypothyroidism.
These hormone levels, combined with TPO antibody results, a patient’s symptoms like fatigue, weight gain, or cold intolerance, and a clinical examination, contribute to a complete diagnosis. The presence of TPO antibodies in a person with normal TSH levels may indicate an increased risk of developing overt hypothyroidism over time. Consulting a healthcare professional is important for accurate interpretation of all test results and diagnosis.