When to Check TPO Antibodies
TPO antibodies should be checked at the time of type 1 diabetes diagnosis, and can be considered in any patient with suspected autoimmune thyroid disease or high-normal TSH levels (2.5-5.49 IU/ml) to identify those at risk for developing hypothyroidism. 1
Primary Indication: Type 1 Diabetes
In patients with type 1 diabetes, check TPO antibodies (along with thyroglobulin antibodies) soon after diagnosis. 1
- Autoimmune thyroid disease occurs in 17-30% of patients with type 1 diabetes, making it the most common associated autoimmune disorder. 1
- Approximately 25% of children with type 1 diabetes have thyroid autoantibodies at diagnosis. 1
- TPO antibodies are more predictive of thyroid dysfunction than thyroglobulin antibodies in multivariate analysis. 1
- The presence of thyroid autoantibodies predicts future thyroid dysfunction—most commonly hypothyroidism (though hyperthyroidism occurs in ~0.5% of cases). 1
Important Timing Consideration
- Thyroid function tests may be misleading if performed at initial diabetes diagnosis due to euthyroid sick syndrome from previous hyperglycemia, ketosis, or weight loss. 1
- Measure TSH after metabolic control has been established or when clinically stable, then recheck every 1-2 years if normal. 1
Secondary Indications: General Population
High-Normal TSH Levels
Consider checking TPO antibodies in patients with TSH between 2.5-5.49 IU/ml to identify those at increased risk of progression to overt hypothyroidism. 2
- The prevalence of positive TPO antibodies is 18.6% in patients with high-normal TSH (2.5-5.49 IU/ml) versus only 3% in those with low-normal TSH (0.36-2.49 IU/ml). 2
- Patients with positive thyroid antibodies have a 4.3% per year risk of developing overt hypothyroidism versus 2.6% per year in antibody-negative individuals. 3
Suspected Autoimmune Thyroid Disease
Check TPO antibodies when evaluating for Hashimoto's thyroiditis or Graves' disease. 4, 5
- TPO antibodies have 96% sensitivity for Hashimoto's thyroiditis and 59% sensitivity for Graves' disease (using a cutoff of 200 units/ml). 5
- Clearly elevated anti-TPO values (>500 units/ml) are found in 59% of patients with thyroiditis but in none of the healthy controls. 5
- The highest frequency of positive results (88%) occurs in patients with autoimmune hypothyroidism. 5
Clinical Scenarios Warranting Testing
Check TPO antibodies in patients presenting with: 4
- Goiter evaluation 4
- Subclinical thyroid disease to predict risk of progression to overt hypothyroidism 4
- Symptoms suggestive of thyroid dysfunction with borderline TSH 1
- Unexplained glycemic variability in diabetic patients 1
Key Clinical Pitfalls
- Do not check TPO antibodies at the time of acute illness or metabolic derangement, as thyroid function tests may be unreliable due to euthyroid sick syndrome. 1
- TPO antibodies identify autoimmune etiology but do not require treatment if thyroid function is normal—the primary value is risk stratification for future dysfunction. 3
- Once positive TPO antibodies are identified, regular monitoring of thyroid function (TSH, free T4) every 6-12 months is essential. 3
- Treatment with levothyroxine is indicated if TSH rises above 10 mIU/L or if symptoms of hypothyroidism develop, not based on antibody levels alone. 3, 6