Measurement of serum thyroglobulin (Tg) by immunoassay is used to monitor the treatment of differentiated thyroid cancer. An understanding of the disease process and the biochemistry of Tg allows the laboratory to determine the characteristics of suitable Tg assays. There are a number of key analytical issues: standardization, long-term assay stability, minimum detection limit, working range and the effect of endogenous Tg antibodies (TgAb). There are significant limitations to the methods used to determine TgAb interference. Clinicians should be aware of the limitations of immunoassays for this tumour marker.