Critical evaluation of serum thyroglobulin levels and I-131 scans in post-therapy patients with differentiated thyroid carcinoma: concise communication.

Serum thyroglobulin measurements by radioimmunoassay were performed in the follow-up of 68 patients with differentiated thyroid carcinoma undergoing I-131 total-body scans following surgery and/or I-131 therapy. Of 12 patients with distant metastases demonstrated by I-131 scan, thyroglobulin levels were elevated (greater than 60 ng/ml) in nine (75%); the remaining 25% either ranged between 20 and 60 ng/ml or were below 20 ng/ml in spite of having functional metastases. Of six patients with only regional lymph-node metastases demonstrated by I-131 scan, only one (16%) had an elevated thyroglobulin level, while two fell in the 20-60 ng/ml range and three were below 20 ng/ml. Of the remaining patients with no metastatic disease demonstrable by I-131 scan, three (6%) had elevated thyroglobulin levels. These patients were subsequently found to have metastatic disease by other criteria. These results suggest caution in the use of thyroglobulin levels as a replacement for I-131 scans in the follow-up of differentiated thyroid carcinoma. Based on our study, however, the two methods complement each other to achieve maximum sensitivity and reliability.