Significance of Washout Thyroglobulin Measurement in Detecting Thyroid Cancer Metastasis

Background/Objectives: The global incidence of thyroid cancer has been increasing, necessitating improved diagnostic strategies for detecting lymph node metastases. Fine-Needle Aspiration Biopsy (FNA) is a widely used diagnostic tool; however, its accuracy is sometimes limited, particularly in cases with non-diagnostic results. Washout Thyroglobulin (Washout Tg) measurement has emerged as an important adjunctive tool in refining thyroid cancer diagnosis. Methods: This retrospective study analyzed 723 patients who underwent thyroid cancer surgery at Samsung Medical Center from 2013 to 2023. The patients were categorized based on their thyroid status into three groups: 1. total thyroidectomy with modified radical neck dissection (mRND); 2. completion thyroidectomy with mRND; and 3. mRND or selective neck dissection (SND) without thyroidectomy. The Washout Tg levels and their diagnostic performance were evaluated using Receiver Operating Characteristic (ROC) analysis, determining the optimal cutoff values for predicting lymph node metastasis. Results: Washout Tg demonstrated high sensitivity for detecting metastases, with the optimal cutoff values varying based on thyroid status. For the patients who had total thyroidectomy, the cutoff was 23.3 ng/mL (AUC = 0.85, sensitivity = 82.6%, and specificity = 75.0%). In completion thyroidectomy cases, a threshold of 7.2 ng/mL (AUC = 0.879) achieved 98.4% sensitivity and 80.0% specificity. For patients without thyroidectomy (mRND/SND group), a cutoff of 0.1 ng/mL (AUC = 0.766) yielded 98.9% sensitivity but lower specificity (60.0%). Additionally, the Washout Tg/serum Tg ratio demonstrated high diagnostic accuracy with a cutoff of >1 (sensitivity = 97.09% and specificity = 63.64%). Conclusions: The Washout Tg measurement and the Washout Tg/serum Tg ratio play a crucial role in detecting lymph node metastases, particularly in patients post-thyroidectomy. The findings emphasize the necessity of adjusting the Washout Tg cutoff values based on thyroid status to enhance diagnostic accuracy. Further prospective studies are required to validate these thresholds and optimize their clinical application.