Anti-thyroid peroxidase (anti-TPO) antibodies in thyroid diseases, non-thyroidal illness and controls. Clinical validity of a new commercial method for detection of anti-TPO (thyroid microsomal) autoantibodies

Abstract

The identification of the thyroid peroxidase (TPO) as the main antigen of the thyroid microsomal fraction has enabled the development of a sensitive and specific assay for detection of the corresponding autoantibodies. We evaluated the diagnostic validity of the anti-TPO assay in 303 patients with different types of thyroid disease and in controls. Clearly elevated anti-TPO values (anti-TPO > 500 units/ml) were found in 59% of patients with thyroiditis but in none of the controls or the patients with non-thyroidal illness. The mean anti-TPO levels in these two control groups were 26 ± 31 units/ml (mean ± S.D.) and 39 ± 34 units/ml, respectively. The highest frequency of positive results (88%) was obtained in patients with autoimmune hypothyroidism (clinical diagnosis: Hashimoto's thyroiditis) followed by patients with Graves' disease (53%). With a cut-off point of 200 units/ml, a sensitivity of 96% was obtained for Hashimoto's thyroiditis and of 59% for Graves' disease with a specificity of 100% (50 cases). The new method (anti-TPO, Dynotest) was compared with three conventional methods (35 samples). The results for all measurements were in general agreement. In two cases the results were clearly discordant: one sample contained high anti-thyroglobulin antibody concentrations, the other was obtained from a patient with non-thyroidal illness. In both instances the ‘classical’ assays yielded false-positive results. Treatment of autoimmune hyperthyroidism resulted in a median decrease in anti-TPO levels of over 50% after reaching the euthyroid state ( P < 0.05), whereas in persistent hyperthyroidism no consistent changes were observed. In autoimmune hypothyroidism a marked variability in anti-TPO levels was noted. Some patients showed a clear decrease in anti-TPO levels during T4 substitution whereas in others no consistent changes were observed.