The test is most sensitive for detection of thyroid cancer recurrence when patients are off thyroid replacement long enough to have an elevated TSH prior to drawing the specimen. This test also can be used to follow patients with normal TSH; however, Tg values from specimens with high TSH should not be compared with values with normal TSH, because TSH stimulation changes the baseline determinations.
Thyroid autoantibodies may interfere with the measurement of Tg. All specimens are prescreened for antibodies and a comment appended to the report if they are present. Undetectable levels of Tg should be interpreted with caution if anti-Tg is present. A Tg antibody result of <22 IU/mL is unlikely to cause clinically significant Tg assay interference. It is recommended that the thyroglobulin result be reviewed for concordance with clinical presentation.
Specimens with Tg concentrations >250,000 ng/mL may hook and appear to have markedly lower values.
Anti-Tg values determined by different methodologies might vary significantly and cannot be directly compared with one another. Some patients might show to be antibody-positive by some methods and antibody-negative by others. Comparing anti-Tg antibodies values from different methods might lead to erroneous clinical interpretation.
In patients receiving therapy with high biotin doses (ie, >5 mg/day), no specimen should be drawn until at least 8 hours after the last biotin administration.
Tg concentrations >2,000 ng/mL may lead to falsely elevated anti-Tg concentrations.