The effect of [125I]thyroglobulin tracer heterogeneity on serum Tg RIA measurement

Comparative serum Tg RIA studies were used to evaluate the contamination of 125I-19S Tg (B) (670 000 Da) with a smaller partially immunoactive degradation product (C) (530 000 Da). B and C tracers prepared either by enzymic (GO), chloramine T (CT) or Bolton Hunter (BH) iodination methods were tested. B tracers, (either GO or CT), gave consistently higher Tg values vs C tracers at serum Tg levels greater than 30 ng/ml. No difference in values was seen with C tracers of either GO, CT or BH origin. The immunological nonidentity between B and C tracers was shown by nonparallelism between diluted high Tg sera and the Tg RIA standards. Nonparallelism existed above 30 ng/ml with all C tracers irrespective of iodination method and was, in addition, present with CT-B tracers from 3/4 Tg preparations. Only B tracers, prepared by GO or BH, consistently showed adequate parallelism. The ubiquitous nature of C contamination of B tracers prompted a comparative study of serum Tg RIA values between four different laboratories. Good interlaboratory agreement was shown for Tg values less than 30 ng/ml, whereas there was a 10- to 20-fold difference in values for sera with high Tg levels (greater than 100 ng/ml). The observed/expected ratio of values, in serial dilutions of a high Tg sera, measured in two of the laboratories, suggested that nonparallelism accounted for some interlaboratory differences. Contamination of 125I-19S Tg (B) by its breakdown product C, has potential to lower absolute serum Tg values and produce non-parallelism in diluted high Tg sera which results in aberrantly low Tg RIA values. This problem potentially limits the clinical application and relevance of serum Tg measurements in thyroid cancer patients, especially those with metastases associated with high serum Tg levels.