The reference limits for thyroid antibodies are generally made by measuring thyroid peroxidase and thyroglobulin antibody values in a group of healthy subjects (direct method), as proposed by the National Academy of Clinical Biochemistry.
To define the upper reference limits of thyroid peroxidase and thyroglobulin, by using an indirect method to analyze data from a large number of outpatients that were stored in the information system of a general hospital laboratory.
Thyroid peroxidase and thyroglobulin values from 21 492 patients, who had undergone antithyroid antibody measurements, were retrieved from the laboratory information system; the upper reference limits (in the top 97.5 percentile) were calculated using the indirect Kairisto method, after exclusion of outliers.
The mean upper reference limits for females and males were 15 kIU/L and 9 kIU/L for thyroid peroxidase, and 21 kIU/L and 19 kIU/L for thyroglobulin, respectively. The upper limits showed minimal or no differences in the different age classes in either females or males.
Using a vast population of patients, we demonstrated that the upper limits for thyroid antibodies are much lower than the values obtained with classic, direct methods and that they do not vary in relation to age and sex.