Management of low-risk patients with thyroid carcinoma and detectable thyroglobulin on T4 after thyroidectomy and ablation with iodine-131

Objective: To evaluate the positive predictive value of detectable Tg during T4 therapy (Tg on T4) in patients with thyroid cancer after total thyroidectomy and remnant ablation, discussing the work-up in this situation and the empirical indication of I-131. Patients and methods: Initially, 234 low-risk patients [tumor <= 5cm, completely resected, no extensive extrathyroid invasion (pT4)] submitted to total thyroidectomy and ablation with I-131 (3.7-5.5 GBq) who presented no ectopic uptake on RxWBS were studied. Of these, 23 patients with detectable Tg on T4 (> 1ng/ml) during the first year after initial therapy were selected. Results: Metastases were detected by neck US in 7 patients, by chest CT in 2 and by US and CT in 3. Four of five patients with lung metastases upon CT had a positive RxWBS. Eleven patients with negative US and CT received a new I-131 dose (without DxWBS), and RxWBS showed ectopic uptake in 3 patients. Among the patients with negative RxWBS, 7 remained free of apparent disease and Tg was declining (5 with undetectable Tg on T4 at the end of the study). One patient presented an increase in Tg and FDG-PET was positive for lymph node and bone metastases. Conclusions: All patients with Tg on T4 > 5ng/ml presented apparent disease. In these cases, even when US and CT are negative, the administration of a therapeutic dose of I-131 (without DxWBS) and FDG-PET are recommended. Among patients with detectable Tg on T4 <= 5ng/ml and negative US and CT, only 12% presented ectopic uptake on RxWBS. These cases could be followed up by monitoring Tg on T4, and RxWBS and FDG-PET should only be performed if this marker does not decrease after 1-2 years.