In this task, you are given a passage which has a question and the context. You have to generate an answer to the question based on the information present in the context.

Context: Although serum thyroglobulin (Tg) is an excellent marker for detecting recurrent or persistent differentiated thyroid cancer (DTC), it is unreliable in patients who have positive anti-Tg antibodies. Furthermore, a growing number of patients with DTC have elevated Tg levels but no detectable disease on radioiodine scanning or other imaging studies. The objective of this study was to determine whether a gradient in Tg protein level exists in patients with DTC.', 'Fifteen patients who underwent thyroidectomy and/or lymph node dissection for primary DTC (n = 10 patients) and recurrent or persistent DTC (n = 5 patients). A venipuncture was performed simultaneously from the internal jugular vein adjacent to the tumor and the ipsilateral antecubital vein. Venous Tg protein levels were measured by using a chemiluminescence assay. RESULTS.: The average internal jugular-to-antecubital vein Tg protein ratio was 3.4:1.0 (median Tg ratio, 2.9:1; range, 0.8-62.2). Four patients had positive anti-Tg antibodies but still had a Tg gradient. Tg levels were significantly higher in the adjacent internal jugular vein than in the antecubital vein (P = .0019). The Tg ratio between the internal jugular and antecubital veins was significantly higher in patients with recurrent or persistent DTC than in patients with primary tumors (P = .0196).\Question: Do patients with differentiated thyroid cancer have a venous gradient in thyroglobulin levels?
To the authors' knowledge, this is the first study to document a venous gradient in Tg protein levels in patients with DTC. The findings suggested that venous sampling for Tg may be used to localize DTC in some patients who have high or increasing serum Tg levels but negative radioiodine scans or imaging studies.