We examined the usefulness of serum thyroglobulin (Tg) levels in 196 p
atients with metastatic disease. Of these, 51 patients had a thyroidal
primary (40 differentiated, 7 medullary and 4 undifferentiated), 35 p
atients had a non thyroidal primary and in 110 patients the primary si
te was not known. Serum Tg was raised in 74.5 per cent (38 of 51) pati
ents with carcinoma of the thyroid and in 92.5 per cent (37 of 40) pat
ients with differentiated thyroid carcinoma (DTC). Twelve (34.3%) pati
ents with proved nonthyroidal malignancies and 34 (30.9%) patients wit
h unknown primary origin had elevated serum Tg levels. Of the 110 pati
ents with unknown primary site thyroidal primary in 10 patients (all w
ith elevated serum Tg levels) and nonthyroidal primary in 38 patients
(7 had raised serum Tg levels) could be established. The sensitivity (
for DTC) and specificity (for nonthyroidal primary) of serum Tg estima
tion were 94.0 per cent (47 of 50) and 74.0 per cent (54 of 73) respec
tively and for patients with distant metastases were 100 (29 of 29) an
d 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 di
stant metastases) of an unknown primary site, serum Tg estimation is o
f great value to identify or rule out the involvement of the thyroid a
s the primary organ.