Thyroid Peroxidase Antibody Normal Range
The normal range for thyroid peroxidase antibody (TPO-Ab) is less than 35 IU/mL, with values above this threshold considered positive for thyroid autoimmunity. 1, 2
Established Reference Values
- The standard cutoff point for TPO-Ab positivity is 35 IU/mL, as established by commercial assay manufacturers and validated in clinical studies 2
- Values below 100 IU/mL are generally considered negative in most clinical contexts 3
- Some laboratories may use slightly different cutoffs (e.g., 200 units/mL for enhanced specificity), but 35 IU/mL remains the most widely accepted threshold 1
Clinical Interpretation by TPO-Ab Level
Low-positive range (35-200 IU/mL):
- May be found in 8-10% of healthy individuals without thyroid disease 1, 2
- Can occur in non-autoimmune thyroid conditions (11% prevalence) 4
- Requires correlation with TSH and clinical findings 5, 6
Moderately elevated (200-1300 IU/mL):
- Strongly suggestive of autoimmune thyroid disease 1
- Found in 53-59% of Graves' disease patients 1, 4
- Common in early Hashimoto's thyroiditis 4
Highly elevated (>1300 IU/mL):
- Virtually diagnostic of significant autoimmune thyroid disease 7
- Associated with multifocal disease in thyroid cancer patients with concurrent Hashimoto's thyroiditis 7
- Highest concentrations typically seen in untreated hypothyroid Hashimoto's thyroiditis (ranges up to 90,000 IU/mL reported) 4
Screening Recommendations
For children and adolescents with type 1 diabetes:
- Test for anti-thyroid peroxidase antibodies soon after diabetes diagnosis 5
- If positive, recheck thyroid function every 1-2 years or sooner if symptoms develop 5
- Measure TSH when clinically stable after optimizing glycemia 5
Important Clinical Caveats
- Up to 43% of individuals without personal or family history of thyroid disease may have positive TPO-Ab titers when using the 35 IU/mL cutoff, though most have low-positive values 2
- TPO-Ab assays are more sensitive and specific than older thyroid microsomal antibody tests by passive hemagglutination 4
- Anti-thyroglobulin antibodies can cause false-positive results in older assay methods but do not interfere with modern TPO-Ab RIA tests 4
- Circulating thyroglobulin concentrations (even >10,000 ng/mL) do not affect TPO-Ab measurement 4
- TPO-Ab levels typically decline during levothyroxine treatment (mean 70% decrease after 5 years), but only 16% of patients achieve complete normalization 3