Serum thyroglobulin in the monitoring of differentiated thyroid cancer

Patients with differentiated thyroid cancer (DTC) usually have an excellent prognosis. Following surgical and radioiodine treatment to remove the cancer cells and suppressive doses of levothyroxine, long-term follow-up, including measurement of serum thyroglobulin (Tg) using a sensitive assay is required to detect recurrence. To interpret Tg results clinicians need to know the corresponding serum TSH concentration, have an appreciation of the clearance of Tg from patient serum following various interventions and the limitations of its measurement. The limitations of Tg immunoassay are well described and include potential interference from TgAb. For the majority of patients with DTC who are TgAb-negative, Tg measurement remains the most useful method of follow-up. For the TgAb-positive minority, interference and the possibility of producing erroneous results is a concern. Some assays are less badly affected than others and laboratories are advised to choose their assays carefully. Laboratories have sought to identify interferences using measurement of TgAb, lack of concordance between RIAs and immunometric assays and recovery of added Tg. More recently LC-MSMS assays to quantify Tg have been developed. They are not currently as sensitive as Tg immunoassays and it is likely these assays will, like immunoassays, be limited by Tg heterogeneity and standardization issues, although initial evaluations indicate that they may have value in the clinical setting as a second line test in antibody-positive DTC patients in whom Tg is unmeasurable by immunoassay.