Monitoring TgAb-positive patients with differentiated thyroid cancer

Abstract

Thyroglobulin (Tg) is a glycoprotein hormone uniquely synthesized and secreted by both benign and malignant thyroid tissue. As such, serial Tg measurements have become a cornerstone in the longterm management of patients with differentiated thyroid cancer (DTC). Currently, three different methods exist for the determination of serum Tg values: immunometric assays (IMA), radioimmunoassay (RIA) and liquid chromatography-tandem mass spectrometry (Tg-LC/MS). All assays produce accurate and reproducible Tg levels, the only difference being the sensitivity limits of each assay. However, in the presence of circulating thyroglobulin antibodies (TgAb), the performance of these assays is reduced. Because 25 percent of patients with DTC have circulating TgAb, accurate Tg measurements can be compromised. In DTC patients with documented disease and TgAb, up to 50 percent of serum Tg levels will result as undetectable when measured either by the IMA or Tg-LC/MS while only four percent are undetectable when measured by the RIA. This necessitates prescreening all Tg samples for the presence of TgAb and rerouting samples to the RIA, if available. As the Tg RIA is not widely available, preliminary studies have demonstrated that TgAb can be used as a surrogate tumor marker in patients with DTC. In conclusion, patients with DTC who are TgAb positive, as described above, require alterations in the usual management of DTC.