Abstract: Objective To investigate the predicative value of midkine (MK) as a cancer biomarker for metastatic lesions in differentiated thyroid cancer (DTC) patients with positive thyroglobulin antibodies (TgAb) before the first 131I therapy. Methods MK levels were measured by enzyme- linked immunosorbent assay in 151 recruited DTC patients included in this study according to strict inclusion and exclusion criteria. There were 28 TgAb positive DTC patients with metastases and 123 DTC patients without metastases. The value of pre- 131I-ablative MK to predict metastasis was assessed by receiver operating characteristic (ROC) curves in these two groups of patients. Results MK levels were significantly higher in TgAb positive DTC patients than those in DTC patients without metastases. MK levels showed good diagnostic value, with an area under the curve of 0.856 (P<0.001), and a diagnostic accuracy of 83% at the optimal cut-off value of 550ng/L. Conclusion Results show that MK can potentially be used as a surrogate biomarker for predicting DTC metastases when thyroglobulin is not suitable due to TgAb positivity.

Key words: thyroid neoplasms, thyroglobulin, ROC curve, midkine, differentiated thyroid cancer, thyroglobulin antibodies, 131I treatment