What does the Thyroid Peroxidase (TPO) Antibody test check for?

What Thyroid Peroxidase (TPO) Antibody Tests Detect

The TPO antibody test checks for autoimmune thyroid disease, primarily identifying Hashimoto's thyroiditis and Graves' disease by detecting antibodies against thyroid peroxidase, the key enzyme responsible for thyroid hormone production. 1

Primary Clinical Purpose

TPO antibodies serve as the most sensitive marker for autoimmune thyroid dysfunction:

• TPO antibodies are more predictive than anti-thyroglobulin antibodies for diagnosing autoimmune thyroid disease and predicting progression to hypothyroidism 2, 1
• These antibodies identify the autoimmune etiology of thyroid dysfunction even when thyroid function tests remain normal 1, 3
• The test detects antibodies against thyroid peroxidase, the major autoantigen in autoimmune thyroid diseases and the enzyme critical for thyroid hormone synthesis 4, 5

Specific Conditions Detected

Hashimoto's Thyroiditis

• TPO antibodies are present in approximately 96% of patients with Hashimoto's thyroiditis when using a cutoff of 200 units/ml 4
• The degree of TPO antibody elevation correlates with the extent of lymphocytic infiltration in the thyroid gland 5, 6

Graves' Disease

• TPO antibodies are found in 53-70% of patients with Graves' disease 4, 6
• While TSH receptor antibodies (TRAb) are the hallmark of Graves' disease, TPO antibodies help confirm the autoimmune nature 1

Risk Prediction in Euthyroid Individuals

• Positive TPO antibodies in euthyroid subjects indicate a 4.3% annual risk of developing overt hypothyroidism, compared to 2.6% in antibody-negative individuals 3
• Even with normal TSH levels, TPO antibody presence predicts impending thyroid failure 5
• TPO-positive women with normal TSH have a 2.1% annual risk of developing hypothyroidism 5

Special Populations Requiring Testing

Type 1 Diabetes Patients

• Test for TPO antibodies soon after type 1 diabetes diagnosis, as autoimmune thyroid disease occurs in 17-30% of these patients 2, 1
• Approximately 25% of children with type 1 diabetes have thyroid autoantibodies at diagnosis, with TPO antibodies being the strongest predictor of thyroid dysfunction 2, 3
• Recheck TSH every 1-2 years if initial tests are normal, or sooner with positive antibodies or symptoms 1

Other High-Risk Scenarios

• Patients receiving immune checkpoint inhibitors who develop thyroid dysfunction 1
• Pregnant women or those planning pregnancy to predict first-trimester hypothyroidism and postpartum thyroid dysfunction 5
• Patients on amiodarone, lithium, or interferon-alpha therapy 5
• Individuals with other autoimmune diseases (celiac disease, adrenal insufficiency) 3

Important Clinical Caveats

Timing of Testing Matters:

• Thyroid function tests may be misleading if performed during acute illness, diabetic ketoacidosis, or metabolic derangement 2, 1
• Repeat testing after achieving metabolic stability for accurate interpretation 2, 1

Prevalence in General Population:

• TPO antibodies are present in 12-26% of euthyroid subjects in the general population 5
• Very low titers can be found in virtually all subjects with sensitive assays, but only titers above assay-specific cutoffs represent true risk 5

Monitoring Implications:

• Positive TPO antibodies warrant regular thyroid function monitoring every 6-12 months 3
• Treatment with levothyroxine is indicated when TSH rises above 10 mIU/L or symptoms develop, not for antibody presence alone 3