A single recombinant human thyrotropin-stimulated serum thyroglobulin measurement predicts differentiated thyroid carcinoma metastases three to five years later.

Abstract

Testing for residual differentiated thyroid carcinoma relies heavily upon recombinant human (rh)TSH-stimulated serum thyroglobulin (Tg) levels, but the positive predictive value is often low. Our objective was to determine the accuracy of a single rhTSH-Tg measurement over time. We conducted a prospective follow-up study at the University referral center. A total of 107 differentiated thyroid carcinoma patients were stratified according to their initial rhTSH-Tg as follows: group 1 with Tg less than 0.5 (n = 68), group 2 with Tg of 0.6-2.0 (n = 19), and group 3 with Tg greater than 2 ng/ml (n = 20). Clinical evaluations were conducted over 0.9-5.2 yr as follows: Tg during thyroid hormone suppression (n = 27), after rhTSH (n = 59), and/or after thyroid hormone withdrawal (n = 15). Tumor was identified in one patient in each of groups 1 (1.6%) and 2 (5.5%), and 16 in group 3 (80%), comprising 19 tumor locations: 11 locoregional, two mediastinal, five lung, and one brain. Tumor was found in 81% with an initial or follow-up rhTSH-Tg greater than 2 ng/ml. TSH-stimulated Tg fell spontaneously to less than 0.5 ng/ml in 50% of group 2 and 5% of group 3 over 1.7-5.0 yr. The positive predictive value of the initial rhTSH-Tg greater than 2 ng/ml was 80%, and the negative predictive value was 98%. After retreatment, 100% of group 1, 74% of group 2, and 55% of group 3 had no evidence of tumor (P = 0.0001). 1) A single rhTSH-Tg greater than 2 ng/ml predicts persistent tumor, although no value entirely excludes future recurrence. 2) Repeated TSH-stimulated studies are appropriate for patients at risk of recurrence, especially those with an rhTSH-Tg greater than 1 ng/ml. 3) A single rhTSH-Tg less than 0.5 ng/ml without Tg antibody has an approximately 98% likelihood of identifying patients completely free of tumor, a large group in which TSH suppression to less than 0.1 mIU/liter and frequent imaging and TSH-stimulated Tg testing are unnecessary.