Thyroglobulin antibody (TgAb) testing is performed on all samples. If TgAb is negative, serum thyroglobulin is measured by a sensitive IMA with a relatively rapid turnaround time, generally two to three days. If TgAb is positive, serum thyroglobulin is measured by a sensitive LC/MS-MS method. This method is less prone to interference by TgAb but has a longer turnaround time of four to six days. Use Thyroglobulin (Tg) measurement is intended to aid in monitoring for the presence of orthotropic and/or metastatic thyroid tissues in patients who have had thyroid gland ablation (using thyroid surgery with or without radioactivity). Measurement of thyroglobulin antibody (TgAb) is used to identify patient samples that may be affected by TgAb interference in the measurement of Tg. Quantitative TgAb concentrations can also serve as a surrogate tumor marker for DTC recurrence and for monitoring changes in tumor mass in certain patients. Limitations Any changes in serum Tg concentrations should be interpreted in light of the total clinical presentation of the patient, including clinical history, data from additional testing and other appropriate information. Single measurement of thyroglobulin close to the limit of detection is of minimal value in assessing disease status. Serial determinations are required, and should be referenced to the postsurgical baseline Tg result when possible. Evaluation of increasing Tg levels over time are more clinically important.1,2 Serial thyroglobulin (Tg) and/or thyroglobulin antibody (TgAb) testing on an individual patient should be performed by the same method for reliable interpretation.1,3-5 When a change in Tg method is necessary, it is recommended to reëstablish a new baseline Tg level to then interpret further change over time. It should be noted that the thyroid tumors of some patients fail to secret a detectable Tg concentration or may secrete abnormal Tg isoforms that are not detectable by some assays used to measure Tg.1 Measurement of preoperative Tg levels can provide the clinician with insights regarding the tumors Tg production and secretion and support the utility of postoperative Tg monitoring. As with all two-site "sandwich" immunoassays, some of the analytical limitations of Tg IMA include hook effects, human mouse antibody (HAMA), and anti-Tg interference.6 The Access Thyroglobulin assay does not demonstrate any "hook" effect for concentrations up to 40,000 ng/mL. The Access Thyroglobulin Antibody does not demonstrate any "hook" effect up to approximately 50,000 IU/mL. For samples that are Tg antibody positive, Tg will be measured using a sensitive LC/MS-MS method that is not subject to TgAb interference. Rare amino acid sequence mutations within Tg could potentially cause a false-low result in the Tg LC/MS-MS assay, if the sequence variation occurs within the tryptic peptide measured by the assay or eliminates the tryptic cleavage site.7 In the heterozygote state, the result would be an apparent reduction in Tg concentration by about 50%, while the homozygous state no TG would be detected.