ABSTRACT

Thyroglobulin (Tg) is widely used as a biochemical tumor marker in the post-treatment follow-up of differentiated thyroid cancer patients. Tg measurement is usually done by immunometric analyzes (IMA). However false negativity is the disadvantage of IMA methods in Tg antibody positive patients. Recently, methods using mass spectrometry to detect Tg have been developed to overcome this interference. In all cases for whom Tg test is requested, serum antithyroglobulin autoantibodies (TgAb) should also be measured for the reliability of Tg measurements made by immune analysis. Tg results measured by IMA will not be reliable if samples are positive for TgAb, then Tg should be measured using MS method as a reflex strategy. It is suggested that the RIA method is resistant to TgAb interference and Tg can be measured with this method in lack of possibility performing MS method. However, it is crucial to keep in mind that the radioimmun assay (RIA) method is also susceptible to diverse interferences. If there is no possibility for verification of Tg through RIA or MS method in TgAb positive patients, Tg test should not be used to monitor residue or recurrence. In this case, TgAb is used as a surrogate tumor marker instead of Tg, and interpretation is made according to the change of TgAb levels during the follow-up. TgAb measurement is performed by immunoassay, nevertheless, there are various limitations of these methods. In spite of the internationally recommended standards, which used in measurements of TgAb, the results between methods may show inconsistence with each other due to some matters include the analytical performances of the methods such as sensitivity/specificity, the difference in threshold values and the heterogeneous structure of TgAbs present in cancer patients. In conclusion, TgAb follow-up should be done with the same measurement method; threshold values should be used for evaluating the test results, the results should not be compared with each other and the clinician must be informed about the method alterations. In addition, there may be various interferences that may cause false negativity or false positivity in immune methods used in TgAb measurements as in Tg measurements. Interference investigation for clinically incompatible results, should be performed and the results always evaluated together with the clinical situation.