Objectives/hypothesis: To determine the usefulness of thyroglobulin (Tg) before ablation and the correlation with posttreatment scanning in patients with thyroid carcinoma., Study Design: Prospective., Methods: Tg during hypothyroidism was determined in 212 patients after thyroidectomy and before ablation. The disease stage was based on clinical examination, Tg, posttherapy scanning, other imaging methods, and histologic confirmation in selected cases., Results: One hundred sixty-four patients presented thyroid remnants only, 31 had lymph node metastases, and 17 had distant metastases. Posttreatment scanning showed a sensitivity of 71% for lymph node and of 94.1% for distant metastases. Ectopic uptake was observed in 5.6% of patients with Tg less than 1 ng/mL, in 9% with levels from 1 to 5 ng/mL, in 10.7% with Tg from 5 to 10 ng/mL, and in 51% with Tg greater than 10 ng/mL. The sensitivity of Tg for metastases was 73 and 66.5% and specificity was 73 and 88.4% at a cut-off value of 5 and 10 ng/mL, respectively., Conclusions: Postoperative Tg less than 10 ng/mL rarely show distant metastases and because these metastases were observed in 31% of patients with values greater than 10 ng/mL, this criterion can be selected for the indication of imaging methods, in addition to neck ultrasound (US). For cases with Tg less than 10 ng/mL, US is necessary because even patients with undetectable Tg had lymph node metastases. The same cut-off can be used for the indication of radioiodine therapy even in the absence of any evidence of disease because 51% of the patients with Tg greater than 10 ng/mL showed ectopic uptake on posttreatment scanning.