September 2007

Q: I have recently been diagnosed with a recurrence of papillary thyroid cancer in the right upper lobe of my lung. I have thyroglobulin antibodies. How much of a difference does this make to my blood thyroglobulin levels as far as accuracy? 

A: The measure of thyroglobulin (TG) is a key test that your doctor uses to determine if you have any residual thyroid cancer. This is a protein made only by the normal thyroid gland and by certain thyroid cancer cells. This protein can be used as a tumor marker only if the thyroid is completely removed by surgery (near total or total thyroidectomy) AND radioactive iodine ablation. Without both procedures, the small amount of residual normal thyroid gland (remnant) will make so much more TG than the cancer cells that the TG cannot be used as a marker for tumor recurrence. TG is produced by follicular and papillary thyroid cancer, the most common types of thyroid cancer. TG is not a good marker for either anaplastic or medullary thyroid cancer. Anaplastic thyroid cancers have lost the ability to make TG and medullary thyroid cancer comes from thyroid cells that do not make TG.

TG antibodies make it very difficult to accurately measure TG levels. About 15-20% of thyroid cancer patients have antibodies directed to TG. This antibody will interfere with the modern immunometric assay and cause an artificially LOW reading for TG in the blood. Any level of TG antibodies will make the assay inaccurate. Some specialized laboratories still do the older type of TG assay called radioimmunoassay (RIA) which can give you a falsely HIGH TG reading. If a patient with TG antibodies has a low TG both by immunometric and RIA assays, then it is likely there is no residual tumor. If a patient has a low immunometric TG and a measurable TG by RIA, then imaging tests such as a high frequency ultrasound or MRI exam of the neck or radioactive iodine whole body scan should be done looking for abnormal lymph nodes or masses. If the TG test by RIA and TG antibody levels are stable over time and imaging is negative, then no additional testing is necessary.

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