Value of thyroglobulin to 131I uptake ratio in selection of initial therapy dose of 131I in patients with differentiated thyroid carcinoma

Aim To test the hypothesis that the ratio of thyroglobulin (Tg) to I-131 uptake in the thyroid bed during the immediate post-thyroidectomy work-up could be used before first I-131 treatment to detect patients with residual or metastatic thyroid cancer and justify the administration of a higher ablation dose in selected cases and a possibly better therapeutic effect. Methods. We retrospectively studied 293 patients with differentiated thyroid carcinoma that received their first I-131 treatment in our department. Patients with Tg > 100 ng/ml I-131 uptake > 10% and measurable Tg-specific autoantihodies, were excluded. According to the post-therapy total body scan (TBS), we divided them in 2 groups: group I, without metastases (negative TBS), and group II, with metastases (positive TBS). The ratio of Tg to I-131 uptake measured before the first I-131 treatment was calculated in both groups. Results. A total of 248 patients were included in the study;, 225 in group I and 23 in group II. Tg to I-131 uptake ratio was significantly lower in group I (mean 2.17 ng/ml/%, range 0-36), than in group II (mean 32.7 ng/mL/%, range 2.14-220), (P < 0.01). The sensitivity, specificity and accuracy (using a threshold ratio 7 ng/mL/% as normal) were all 95.6% for predicting a positive post-therapy TBS. Conclusion The use of a threshold ratio 7 ng/mL% as the upper limit of normal provides usefid information with higher sensitivity and specificity in identifying patients with metastatic disease creating the possibility for the selective use of higher initial iodine therapy doses.