Thyroid Research Thyroid Research Archive Thyroid Cancer

Different serum thyroglobulin assays may give discordant results in patients with thyroid carcinoma

(November 2003)

The background of the study. Serum thyroglobulin is often measured during follow-up of patients with thyroid carcinoma, because high values are a sensitive indicator of recurrent carcinoma. However, the results may vary according to the assay used for the measurements. In this study, serum thyroglobulin was measured by two methods in patients with thyroid carcinoma.

How the study was done. Serum thyroglobulin was measured by a radioimmunoassy (RIA) and an immunoradiometric assay (IRMA) in 82 patients with thyroid carcinoma. Assay results were considered discordant when the result in one of the assays was undetectable, and the result in the other was >1 µg/L. Patients with detectable serum thyroglobulin values in either assay were evaluated for persistent or recurrent carcinoma.

The results of the study. All 82 patients had been treated, and all were taking high doses of thyroxine. The serum thyroglobulin values were concordant in 66 patients (80 percent), 10 of whom had values >1 µg/L and 56 had undetectable values.

Four patients (5 percent) had serum thyroglobulin values <1 µg/L, as measured by IRMA, and values ranging from 7.2 to 45 µg/L, as measured by RIA. One patient had recurrent carcinoma at that time, and it was detected later in another.

Twelve patients (15 percent) had serum thyroglobulin values <1 µg/ml, as measured by RIA, and values ranging from 1.4 to 360 µg/ml, as measured by IRMA. Three patients had evidence of recurrent carcinoma, and nine did not; seven of these nine patients later had recurrent carcinoma.

The conclusions of the study. In patients with thyroid carcinoma, serum thyroglobulin concentrations measured by different methods usually give similar results. When the results are dissimilar, they are more likely to be falsely undetectable when measured by RIA.

The original article. Weightman DR, Mallick UK, Fenwick JD, Perros P. Discordant serum thyroglobulin results generated by two classes of assay in patients with thyroid carcinoma: correlation with clinical outcome after 3 years of follow-up. Cancer 2003;98:41-7.