PURPOSE: Postoperative serial serum thyroglobulin (Tg) measurements on levothyroxine (L-T4) therapy in patients with differentiated thyroid carcinoma (DTC) is known to be useful in monitoring tumor progression or regression. The objective of this study was to evaluate the significance of serum Tg levels on L-T4 therapy after surgery. METHODS: To determine the basal serum Tg levels on L-T4 therapy after surgery in patients with DTC, Tg levels during the initial 2 year-period after surgery were analyzed retrospectively in 37 patients who had undergone a total thyroidectomy for papillary thyroid carcinoma with (Group 2) or without (Group 1) palpable cervical lymph node metastasis. The Tg levels had been measured 1 to 4 times for each patient with total of 53 and 35 times in Group 1 and Group 2 respectively. RESULTS: The basal Tg levels in Group 1 were all less than 4 ng/mL with 79.3% less than 2 ng/mL, and those in Group 2 were all less than 9 ng/mL with 71.4% less than 2 ng/mL. CONCLUSION: The data suggests that the postoperative serum Tg level of 10 ng/mL can be a useful reference value in long-term follow-up after total thyroidectomy for patients with DTC.