Serum thyroglobulin level after radioiodine therapy (Day 3) to predict successful ablation of thyroid remnant in postoperative thyroid cancer.

OBJECTIVE: The utility of serum thyroglobulin (Tg) level, 3 days after radioactive iodine (RAI) therapy, was assessed as a means of predicting successful ablation of thyroid remnant in patients with postoperative thyroid cancer.

METHODS: A total of 152 patients with thyroid cancer (mean age = 44.9 ± 13.7 year) undergoing RAI therapy after total thyroidectomy were included. Levels of TSH-stimulated Tg prior to ablation (stimTg) and serum Tg sampled immediately after RAI therapy (Day 3) were measured (immTg). ImmTg samples were collected during patient hospital visits for scheduled follow-up of radioiodine scans. Successful ablation was determined by the second time stimulated Tg levels (≤1 ng/ml) and negative radioiodine uptake at thyroid bed after 6.1 ± 1.1 months of RAI therapy. Univariate and multivariate analyses were done for immTg, stimTg, change in Tg levels (deltaTg: immTg - stimTg), immTg:stimTg ratio (ratioTg), and other potential clinical and pathologic markers of successful ablation.

RESULTS: Of selected laboratory variables, ratioTg was a significant predictor of successful ablation. StimTg, tumor diameter, metastatic lymph node (LN) numbers, lymphatic invasion were possible clinical markers of successful ablation by univariate analysis. By multivariate analysis, ratioTg (odds ratio = 7.851), stimTg (odds ratio = 16.819), metastatic LN numbers (odds ratio with stimTg = 6.732) proved significant results. Furthermore, combining high ratioTg and low stimTg provided added predictive value.

CONCLUSIONS: High ratioTg (reflecting extensive release of Tg to the blood after RAI therapy) and low stimTg (reflecting small remnant thyroid tissue) constitute the indices of successful ablation after RAI therapy. Immediate Tg level could give an useful information on RAI ablation of postoperative thyroid remnant.