Background: Serum thyroglobulin (Tg) is a reliable tumor marker in patients with differentiated thyroid carcinoma (DTC). Distant metastases of DTC, particularly in the lung, normally presents with higher levels of Tg, which is useful to follow the disease evolution. We describe a patient with DTC with lung metastases, with undetectable Tg and Tg-antibodies (TgAb).
Clinical case: A 52-year-old woman underwent a subtotal thyroidectomy in 1993 because of a growing nodule. The histology was follicular thyroid carcinoma. Eight years later, multiple lung nodules were identified in a thorax-CT scan. A biopsy of one of these lesions was compatible with thyroid carcinoma metastases. The patient was referred to our hospital. The thyroidectomy was completed and seven radioiodine treatments were administered (total activity of 800 mCi). The tail-end-scan following each treatment showed uptake in the lung metastases, despite the undetectable stimulated-Tg throughout these treatments. During the 13-years follow-up period, the suppressed-Tg has been consistently undetectable and the measurement of TgAb was always negative (tested by different laboratorial methods), despite the progression in size and number (some with ~3 cm) of the known fluorodeoxyglucose (FDG-F18)-avid lung lesions. Currently, the patient is asymptomatic with a progressive metastatic DTC, without biochemical evidence of the disease undetectable Tg and TgAb (assessed by different methods) as mentioned above.
Discussion: Serum Tg is an excellent tumor marker for DTC, and serial monitoring is valuable for the follow-up of this condition. DTC with systemic dissemination normally has measurable Tg. Undetectable serum Tg in recurrent/metastatic DTC may not necessarily predict neither radioiodine uptake nor adverse prognosis in these cases, and it imposes important diagnostic, management and follow-up challenges. In such uncommon cases, imaging methods (CT, MR and PET) should supplement Tg and TgAb measurements in order to permit a correct follow-up and management strategy.