Clinical utility of serum thyroglobulin in metastatic disease

We examined the usefulness of serum thyroglobulin (Tg) levels in 196 patients with metastatic disease. Of these, 51 patients had a thyroidal primary (40 differentiated, 7 medullary and 4 undifferentiated), 35 patients had a nonthyroidal primary and in 110 patients the primary site was not known. Serum Tg was raised in 74.5 per cent (38 of 51) patients with carcinoma of the thyroid and in 92.5 per cent (37 of 40) patients with differentiated thyroid carcinoma (DTC). Twelve (34.3%) patients with proved nonthyroidal malignancies and 34 (30.9%) patients with unknown primary origin had elevated serum Tg levels. Of the 110 patients with unknown primary site thyroidal primary in 10 patients (all with elevated serum Tg levels) and nonthyroidal primary in 38 patients (7 had raised serum Tg levels) could be established. The sensitivity (for DTC) and the specificity (for nonthyroidal primary) of serum Tg estimation were 94.0 per cent (47 of 50) and 74.0 per cent (54 of 73) respectively and for patients with distant metastases were 100 (29 of 29) and 85.1 per cent (40 of 47) respectively. The positive and the negative predictive values of serum Tg were 71.2 (47 of 66) and 94.7 per cent (54 of 57) respectively and for patients with distant metastases were 80.6 (29 of 36) and 100 per cent (40 of 40) respectively. Our findings suggest that for patients presenting with metastases (particularly distant metastases) of an unknown primary site, serum Tg estimation is of great value to identify or rule out the involvement of the thyroid as the primary organ.