Thyroid Peroxidase Antibody Unit Conversion
A Thyroid Peroxidase Antibody (TPO Ab) level of < 1.00 IU/mL is equivalent to < 1.00 mIU/L, as these units are identical in measurement of TPO antibodies.
Understanding the Units
The units IU/mL and mIU/L are equivalent for measuring TPO antibodies:
- IU = International Units
- mIU = milli-International Units
- 1 IU/mL = 1000 mIU/mL
- 1 mIU/L = 1 mIU/1000 mL = 0.001 mIU/mL = 0.001 IU/mL
However, when expressing the same concentration:
- 1 IU/mL = 1 mIU/L (because the volume conversion cancels out the milli prefix)
Clinical Significance of TPO Antibody Levels
TPO antibodies are important markers for autoimmune thyroid disease. According to the literature:
- Normal subjects typically have TPO antibody levels below 52 U/mL 1
- In the study by Henning Berlin, the majority of normal subjects had anti-TPO levels below 52 U/mL 1
- A level of < 1.00 IU/mL is well within normal range and indicates absence of thyroid autoimmunity
Interpretation in Clinical Context
- TPO antibodies are a key marker for autoimmune thyroid diseases such as Hashimoto's thyroiditis and Graves' disease 2
- Patients with Hashimoto's thyroiditis typically have levels above 200 U/mL 3
- Even within the normal TSH range, TPO antibody titers correlate with TSH levels, suggesting their presence may herald impending thyroid failure 4
Important Considerations
- Modern TPO antibody assays are highly sensitive, and very low titers can be found in virtually all subjects 4
- The clinical cut-off for significance varies by assay but is typically 200-500 U/mL 3
- A value of < 1.00 IU/mL is significantly below any clinical threshold for concern
Monitoring Recommendations
For patients with normal TPO antibody levels (like < 1.00 IU/mL):
- No specific monitoring is required for the antibody itself
- Regular thyroid function monitoring may be appropriate if there are other risk factors for thyroid disease
For reference, the normal TSH reference range is 0.45 to 4.5 mIU/L according to NHANES III data 5