Potential value of the serum thyroglobulin alteration after radioiodine therapy for tissue characterization of iodine uptake site

Objectives Focal iodine uptakes on post-therapeutic whole body scan (WBS) had a limitation for the characterization of different thyroid tissues. We investigated whether the elevation of serum Tg level after radioiodine therapy (RIT) could be used for the characterization of remnant thyroid tissue on WBS in patients with differentiated thyroid cancer (DTC).

Methods Among the patients who underwent surgery and RIT aided by recombinant human TSH (rhTSH), we included the patients who had iodine uptakes only in both thyroid operation bed and midline of anterior neck on WBS. They were classified into two groups according to the predominance of iodine intensity between two uptake sites: the dominant uptake in midline uptake (group 1) and the dominant uptake in thyroid operation bed (group 2). Serum Tg level was measured twice; 2 days after RIT (D2Tg) and 7 days after RIT (D7Tg). We compared the level of D2Tg, D7Tg and ratioTg (D7Tg/D2Tg) between two groups, respectively. We excluded the patients who had the intense uptake difficult to distinguish the iodine uptake sites, Tg antibody 蠅 60 U/mL or distant metastasis.

Results Total 35 patients were enrolled in this study: group 1 (23 patients, 65.7%) and group 2 (12 patients, 34.3%). Although there was not significantly different in D2Tg between two groups (group 1 vs. group 2: 2.10±1.48 vs. 1.82±1.51; P=0.482), ratioTg of group 2 was significantly higher than that of group 1 (3.91±4.43 vs. 10.16±10.86; P=0.016). There were no significantly differences in TgAb or TSH levels at each time point between two groups.

Conclusions The dominant uptake in thyroid operation bed was closely related to the elevation of serum Tg level after RIT, compared with dominant midline uptake. Our preliminary study had a new insight that tissue characteristic of iodine uptake site (such as cellular damage by radioiodine) might be evaluated with the alteration of serum Tg after RIT. Further study is necessary to evaluate the relationship between the change of iodine accumulation over time and elevation of serum Tg level after RIT.