Correlation between remnant thyroid gland I-131 uptake and serum thyroglobulin levels: can we rely on I-131 whole body scans?

Background I-131 treatment (RAI) decision relies heavily on serum thyroglobulin (Tg) levels, as higher Tg levels are assumed to be correlated with higher I-131 uptake. Tg elevation, negative iodine scintigraphy (TENIS) definition is becoming more clinically relevant as alternative treatment methods are available. This study examined the correlation between Tg levels with I-131 uptake in remnant thyroid gland to evaluate the reliability of serum Tg levels in predicting I-131 uptake. Methods From March 2012 to July 2019, 281 papillary thyroid cancer patients treated with 150 mCi RAI were retrospectively enrolled. Early (2nd day) and Delayed (7th day) post-RAI whole-body scan (WBS) neck counts were correlated with clinical and pathologic findings. Patients with normal neck ultrasound and undetectable level of serum Tg (< 0.2 ng/mL) and thyroglobulin antibody (TgAb) (< 10 IU/mL) were defined as ablation success within 2 years after I-131 ablation. Results Thyroid gland weight, tumor size and thyroiditis were independent factors of preoperative serum Tg levels. Serum off-Tg levels correlated with Early and Delayed WBS neck counts, and thyroiditis pathology contributed to lower neck counts in both Early and Delayed WBSs. In multivariable analysis, Delayed WBS neck count, serum off-Tg and off-TgAb were significant factors for predicting ablation success. Conclusion I-131 uptake and retention in remnant thyroid gland correlates with serum off-Tg levels, thyroiditis, and ablation success in thyroid cancer patients receiving high-dose I-131 therapy. Semi-quantitative I-131 analysis with Early and Delayed WBSs provides additional information in evaluating ablation success, with the potential application for metastasis treatment response evaluation.