Residual Thyroid Tissue on Postoperative Diagnostic 131 I Radioactive Whole-Body Scan After Surgery in Differentiated Thyroid Cancer: A Tertiary Referral Centre Experience

Residual thyroid tissue after total thyroidectomy in differentiated thyroid cancers is considered an independent risk factor for recurrence. Guidelines recommend following up patients after surgery with thyroglobulin (Tg), neck ultrasonography, and occasionally whole-body radioactive scan. However, the results of serum thyroglobulin and whole-body radioiodine scan are often discordant. The present study was undertaken to determine the levels of serum-stimulated thyroglobulin to complement the findings of residual thyroid tissue in the radioactive whole-body scan. One hundred twenty-six patients had undergone a radioiodine (131 I) whole-body scan (WBS) during the study duration, and 121 were available for analysis. The thyroglobulin level (measured by the CLIA method) was recorded at the time of these scans. The data was analysed to determine the level of stimulated thyroglobulin correlating with residual thyroid tissue, locoregional, and distant metastasis as assessed by WBS. The presence of residual thyroid tissue was noted in an overwhelmingly high 94% of cases. Twenty-four of the 28 patients with stimulated Tg < 2 ng/dl had residual thyroid tissue on a WBS. The discordancy rate (positive moderate — large WBS and negative serum thyroglobulin) of 64.28% was seen. Using ROC the serum thyroglobulin cut-offs levels for the loco-regional disease were found to be 27.705 ng/dl and 94.770 ng/dl for distant metastasis. The results highlight the fact that serum Tg levels cannot be used as an accurate predictor of the extent of the remnant thyroid tissue. Irrespective of the quality of surgery, which was analysed based on the centre and surgical specialty, over 90% of cases had residual thyroid tissue on WBS. The use of only stimulated Tg levels for follow-up may be inaccurate. Serum Tg is a useful test along with radioactive whole-body scans to distinguish local disease, loco-regional disease, and distant metastasis.