Blood Test Specific to Hashimoto's Thyroiditis
Anti-thyroid peroxidase (anti-TPO) antibodies are the most specific blood test for Hashimoto's thyroiditis, with a sensitivity of 96% and specificity of 100% when using a cutoff of 200 units/ml. 1
Primary Diagnostic Antibodies
Anti-TPO Antibodies (Most Specific)
- Anti-TPO antibodies are present in 75-96% of patients with histologically confirmed Hashimoto's thyroiditis, making them the primary diagnostic marker 1, 2
- Using a cutoff of 200 units/ml, anti-TPO testing achieves 96% sensitivity and 100% specificity for Hashimoto's thyroiditis 1
- Clearly elevated anti-TPO values (>500 units/ml) are found in 59% of Hashimoto's patients but in none of the healthy controls 1
- The highest frequency of positive anti-TPO results (88%) occurs in patients with autoimmune hypothyroidism (clinical diagnosis: Hashimoto's thyroiditis) 1
Anti-Thyroglobulin Antibodies (Complementary Test)
- Anti-thyroglobulin (TgAb) antibodies are actually more sensitive than anti-TPO for detecting Hashimoto's thyroiditis in many cases, with prevalence of 96-98.6% versus 73.5-81.4% for anti-TPO 3, 2
- TgAb by radioimmunoassay is positive in 96.4% of patients with histologically proven Hashimoto's thyroiditis 2
- In patients with negative conventional hemagglutination tests, TgAb positivity by radioimmunoassay is more closely associated with histological diagnosis of Hashimoto's thyroiditis (89.7%) than TPOAb positivity (27.6%) 2
Recommended Testing Algorithm
Initial Screening
- Measure both anti-TPO antibodies and anti-thyroglobulin antibodies, as either can be positive in Hashimoto's thyroiditis 2, 4
- Anti-TPO should be the first-line screening test due to its high specificity, but TgAb should be added when anti-TPO is negative but clinical suspicion remains high 3, 2
- Thyroid function tests (TSH and free T4) should accompany antibody testing to assess functional status 5
Interpretation of Results
- Anti-TPO >200 units/ml strongly suggests Hashimoto's thyroiditis with 96% sensitivity and 100% specificity 1
- Positive anti-TPO antibodies identify an autoimmune etiology and predict higher risk of progression to overt hypothyroidism (4.3% per year versus 2.6% in antibody-negative individuals) 6
- The combination of elevated TSH with positive anti-TPO antibodies confirms autoimmune hypothyroidism (Hashimoto's thyroiditis) 6
Important Clinical Caveats
Variant Presentations
- Multinodular Hashimoto's thyroiditis shows lower prevalence of elevated anti-TPO antibodies (36.9%) compared to diffuse Hashimoto's (77.7%), suggesting a variant etiopathogenesis 7
- In multinodular presentations, the absence of elevated anti-TPO does not exclude Hashimoto's thyroiditis 7
False Positives to Avoid
- High anti-thyroglobulin antibody concentrations can produce false-positive results in conventional anti-microsomal antibody assays 1
- Thyroid antibodies are found in the general population at baseline rates, so clinical context is essential for interpretation 5
- Anti-TPO antibodies can be present in other conditions including Graves' disease (53-80.8% positive) and painless thyroiditis (33%) 3, 1