Autoantibodies to thyroperoxidase (TPOAb) in a large population of euthyroid subjects: implications for the definition of TPOAb reference intervals.

Objective: Measurement of autoantibodies against thyroperoxidase (TPOAb) plays an important role in the diagnosis of autoimmune thyroid disease. The assessment of reference intervals for TPOAb, however, is a controversial issue since elevated TPOAb values are sometimes found in subjects without other evidence of thyroid disease.

Methods: TPOAb were measured in 1,295 euthyroid individuals using a highly sensitive, fully automated chemiluminescence assay (Advantage A-TPO, Nichols Institute Diagnostics, CA, USA). The study subjects participated in a population study on the prevalence of thyroid disorders in the German federal state of Saxony, an area of mild iodine deficiency.

Results: TPOAb above the detection limit of 0.45 IU/ml were found in 1,277/1,295 euthyroid individuals. TPOAb values in the low measurable range below 1.1 IU/ml followed a normal distribution, and this was independent of age and sex. When using a cut-off value of 1.1 IU/ml, which corresponds to a sensitivity of 79% and a specificity of 95% resulting from the receiver-operator characteristic plot for discrimination between a main type and other types with a higher mean value of TPOAb, elevated TPOAb were found in 14.4% of euthyroid men and in 25.8% of euthyroid women.

Conclusions: The results demonstrate for the first time that TPOAb are detectable in nearly all euthyroid individuals and that TPOAb values in the low measurable range are normally distributed. The distribution of TPOAb values in the low range is independent of age and sex. Based on these data, reference intervals for TPOAb can be defined that are independent of the population investigated. The clinical significance of slightly elevated TPOAb, however, has still to be defined by prospective studies.