Pre-surgical serum Tg values for differentiated thyroid cancer are not meaningful for diagnosis because blood Tg can be elevated in patients with thyroid disease who do not have thyroid cancer, while blood Tg can be normal in patients with thyroid cancer. Pre-surgical blood Tg levels in differentiated thyroid cancer are positively correlated with tumor size. The biological half-life of Tg in vivo is 65.2 h, and it takes 5-10 d after thyroidectomy for Tg to fall below 5-10 μg/L. Ronga et al [6] retrospectively analyzed 334 patients with differentiated thyroid cancer and measured blood Tg for the first time at 40 d after surgery and followed up with regular blood Tg measurements and whole-body scans for 4-16a. As a result, the first blood Tg values were significantly higher in 79 patients with tumor metastases during 18 months after surgery (258.9±31.1) than in patients without metastases (15.9±19.6) μg/L, p<0.0001]. Therefore, positive blood tg after surgery suggests tumor recurrence or metastasis. After total thyroidectomy and high-dose 131I therapy in patients with differentiated thyroid cancer, if serum TGAb is negative, then serum Tg should not be measured. If serum TSH is suppressed, elevated serum Tg is often indicative of remaining tumor tissue or metastases. After total thyroidectomy for papillary and follicular thyroid carcinoma, blood Tg should be <10 μg >10 μg/L to indicate the possibility of metastatic foci. the sensitivity of this diagnosis is 100% and the specificity is over 80%. a negative Tg measurement can reduce unnecessary whole body 131I scans during follow-up.