Iodine Uptake Patterns on Post-ablation Whole Body Scans are Related to Elevated Serum Thyroglobulin Levels After Radioactive Iodine Therapy in Patients with Papillary Thyroid Carcinoma.

Purpose: Serum thyroglobulin (Tg) level is frequently elevated shortly after radioactive iodine (RAI) ablation therapy. The authors studied the relationship between the elevation of serum Tg after RAI therapy and iodine uptake pattern on post-ablation whole body scans (RxWBSs) in patients with papillary thyroid carcinoma (PTC).

Materials And Methods: The study subjects were patients with PTC that had undergone first RAI therapy with thyroid hormone withdrawal after total thyroidectomy. Patients with a high level of serum anti-Tg antibody (TgAb, ≥ 60 U/mL), possible regional or distant metastasis as determined by pre-ablation or post-ablation studies, and negative iodine uptake of the anterior neck on RxWBS were excluded. Serum Tg was checked twice, that is, 7 days after (post-ablation Tg) and on the day of RAI therapy (pre-ablation Tg). Ratio of pre-ablation Tg to post-ablation Tg (Tg ratio) was used to assess changes in serum Tg levels after RAI therapy. Patients were classified into two groups according to the presence of midline uptake above the thyroidectomy bed on RxWBS (negative (group 1) or positive (group 2) midline uptake). Variables were subjected to analysis to identify differences between the two groups.

Results: Two hundred and fifty patients were enrolled in this study; 101 in group 1 and 149 in group 2. Based on univariate analysis, post-ablation Tg (8.12 ± 11.05 vs. 34.12 ± 54.31;  < 0.001) and Tg ratio (7.81 ± 8.98 vs. 20.01 ± 19.84;  < 0.001) were significantly higher in group 2. On the other hand, gender, tumor (T) stage, lymph node (N) stage, size, multiplicity or bilaterality of primary tumor, dose of I, serum TgAb and thyroid-stimulating hormone (TSH) level (before or after RAI therapy) were not significantly different in the two groups. Variables with values of < 0.25 by univariate analysis were subjected to multivariate analysis, which showed post-ablation Tg (OR 1.060, 95 % CI = 1.028-1.092;  < 0.001) and Tg ratio (OR 1.059, 95 % CI = 1.028-1.092;  = 0.001) were significantly higher in group 2.

Conclusion: Serum Tg level after RAI therapy was significantly higher in patients with midline uptake on RxWBS, compared with patients without midline uptake on RxWBS. Further investigations are needed to reveal the correlation between serum Tg elevation and clinical outcome according to the presence of midline uptake.