Assessment of the influence of thyroglobulin (Tg) autoantibodies and other interfering factors on the use of serum Tg as tumor marker in differentiated thyroid carcinoma.

The purpose of the study was to examine the value of a commercial immunoradiometric (IRMA) method for measuring serum thyroglobulin as a tumor marker after treatment for differentiated thyroid carcinoma. A prospective analysis of consecutive serum samples from 53 patients was performed using the IRMA method and a traditional double antibody radioimmunoassay (RIA). The results were compared with those of 100 healthy control subjects and furthermore the method was validated by investigating sera from 24 patients with Hashimoto's thyroiditis positive for thyroglobulin autoantibodies. Finally, in vitro studies of the influence of thyroglobulin autoantibodies on the method were done. The IRMA method had an acceptable analytical precision and was more sensitive than the RIA. It was furthermore less sensitive to the presence of thyroglobulin autoantibodies but it was affected by them, and it showed less unspecific serum effect. Both methods had limitations as tumor marker when the patients had a thyroid remnant, when serum thyrotropin was not suppressed, and in cases of local recurrence. The highest predictive value was found in patients with distant metastases. Thus, in cases of only slightly elevated serum thyroglobulin, the strongest indication for recurrence is still an increasing serum thyroglobulin level within the same patient rather than a single value.