Cautions

Thyroglobulin by Mass Spectrometry:

Rare normal amino acid sequence variations within thyroglobulin (Tg) can cause a false-low result in the Tg mass spectrometry assay if they happen to be present in the Tg proteotypic peptides that are used for Tg quantification. While the exact prevalence of such changes is unknown, the validation data on large sample numbers indicate that this affects less than 1% of samples. In the heterozygote state, the result would be an apparent reduction in Tg concentration by about 50%, while the homozygous state (<0.01%) is predicted to result in total loss of signal. Therefore, if the results of the mass spectrometry measurement are much lower than those obtained previously (within 3-6 months) with an immunometric immunoassay, this possibility should be considered. In this event, alert Mayo Clinic Laboratories as soon as possible, and an attempt will be made to resolve the discrepancy.

Thyroglobulin by Immunoassay:

The test is most sensitive for detection of thyroid cancer recurrence when patients are off thyroid replacement long enough to have an elevated thyrotropin (TSH) prior to collecting the specimen. This test can also 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.

Thyroglobulin autoantibodies (TgAb) may interfere with the measurement of Tg. All specimens are prescreened for TgAb, and a comment appended to the report if they are present. Undetectable levels of Tg should be interpreted with caution if TgAb are present. A Tg antibody result of less than 1.8 IU/mL is unlikely to cause clinically significant Tg assay interference. It is recommended that the Tg result be reviewed for concordance with clinical presentation.

Specimens with Tg concentrations greater than 250,000 ng/mL may "hook" and appear to have markedly lower levels.

Tg and TgAb values determined by different methodologies might vary significantly and cannot be directly compared with one another. Some patients might be antibody-positive by some methods and antibody-negative by others. Comparing values from different methods might lead to erroneous clinical interpretation.

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation.