Value of the first serum thyroglobulin level after total thyroidectomy for the diagnosis of metastases from differentiated thyroid carcinoma.

The aim of this study was to evaluate the diagnostic significance of the first serum thyroglobulin (Tg) measurement, performed 40 days after total thyroidectomy for differentiated thyroid carcinoma and prior to the ablation of residual thyroid tissue by means of iodine-131 therapy. In a retrospective study we examined 334 consecutive patients followed up for 4-16 years by means of regular Tg measurements, (131)I whole-body scans (WBS) and other diagnostic techniques, if necessary. In 79 patients metastases were discovered (32 lymph node and 47 distant metastases) within 18 months following thyroidectomy. Mean values of first Tg were significantly higher in patients with than in patients without metastases (258.9+/-310.6 vs 15.9+/-19.6 ng/ml; P<0.0001). Receiver operating characteristic (ROC) curve analysis of data revealed that for first Tg values higher than 69.7 ng/ml, the positive predictive value for the presence of metastases exceeded 90%. No statistically significant correlation was found between first Tg value and either thyroid-stimulating hormone (TSH) value or percentage of (131)I uptake by residual thyroid tissue. No other parameter (age, histological type, site of metastases, (131)I uptake by metastases) was significantly related to the first Tg value. We conclude that the first Tg measurement after total thyroidectomy provides a useful early diagnostic indication of metastatic disease in spite of the presence of a post-surgical thyroid remnant, and that this holds true regardless of the TSH value and WBS result. This early information is of clinical relevance for patient follow-up.