To explore the relationship between the stimulated thyroglobulin (sTg) and site, number and diameter of metastatic lesions in patients with differentiated thyroid carcinoma (DTC) before the first ¹³¹I treatment, and to evaluate the predictive value of sTg for different metastatic sites.

A total of 567 DTC patients (179 males, 388 females; age: (45.3±12.3) years) who received the first ¹³¹I treatment between January 2012 and June 2017 were included. Thyroglobulin antibody (TgAb), sTg and thyroid stimulating hormone (TSH) were determined within 1 week before ¹³¹I treatment. Metastases were detected by ultrasonography, CT or ¹⁸F-fluorodeoxyglucose (FDG) PET/CT, ¹³¹I whole-body scan, SPECT/CT imaging and pathology. sTg levels of patients with different metastatic sites and different metastasis numbers or lesion diameters were compared (Kruskal-Wallis H test). Spearman correlation analysis was performed on the number, diameter and sTg level of metastases. The receiver operating characteristic (ROC) curve was used to explore the predictive value of sTg before the first ¹³¹I treatment for DTC metastasis.

The median values of sTg in the bone, lung, lymph node metastases groups and non-metastasis group were 500.00, 104.40, 27.45, 2.39 μg/L, respectively, and there were significant differences, except for bone and lung metastases groups (H range: -294.605 to 175.162, all P<0.05). The sTg levels of lung metastasis group and lymph node metastasis group were both decreased by the order of metastasis numbers (≥3, =2, =1; H range: -57.887 to 48.763, all P<0.05). As to the diameter of metastases, the sTg levels of >2.0 cm, 1.1-2.0 cm, and ≤1.0 cm subgroups in the lung metastasis group and lymph node metastasis group were also decreased in order (H range: -69.935 to 61.043, all P<0.05). Spearman correlation analysis showed that the number (r_s=0.568, 0.606) and diameter (r_s=0.806, 0.664) of the metastases in the lung and lymph node metastases group were positively correlated with sTg (all P<0.05). Areas under ROC curves for sTg to predict bone, lung and lymph node metastasis were 0.935, 0.843 and 0.791 respectively. The threshold values were 197.65, 23.21 and 10.96 μg/L respectively. The sensitivities and the specificities were 91.70%, 79.60%, 67.20% and 97.20%, 80.80%, 82.70% respectively.

Tg level before the first ¹³¹I treatment has a certain predictive value for the metastasis, metastatic site and number or diameter in DTC patients.

Thyroid neoplasms; Neoplasm metastasis; Thyroglobulin; Radiotherapy; Iodine radioisotopes