Serum thyroglobulin (Tg) and Tg antibody (TgAb) levels, together with neck ultrasonography and 131I whole-body scintigraphy (WBS), are diagnostic tools for postoperative follow-up of patients with differentiated thyroid carcinoma (DTC). Generally, good correlation is seen between Tg and WBS in follow-up studies for DTC after thyroid remnant ablation. Undetectable serum Tg with negative WBS results suggests complete remission, whereas detectable, or elevated, serum Tg is associated with radioiodine uptake in local or distant metastases. Patients with thyroid cancer cells lacking radioiodine uptake despite an elevated serum Tg level have been referred to as WBS-negative, Tg-positive patients, who represent 10–15% of cases. 18F-FDG PET (FDG-PET) scanning should be considered in high-risk DTC patients with negative WBS and positive Tg. The preferred therapeutic hierarchy for Tg-positive and WBS-negative metastases is surgical excision of loco-regional disease, 131I therapy for radioiodine-responsive disease, external beam radiation, TSH suppression, and systemic therapy with kinase inhibitors. If FDG-PET diagnostic results are negative, one course of 131I treatment may be considered in high-risk patients and individualized. No further 131I therapy is indicated for patients with a negative post-therapy WBS.
Part of the book: Thyroid Cancer