Low serum thyroglobulin level in A 15-year-old girl with papillary thyroid cancer and multiple neck lymph nodes: a case report.

[BACKGROUND] Papillary thyroid carcinoma (PTC) is the most common form of differentiated thyroid cancer. Serum thyroglobulin (Tg) is a key biomarker used in postoperative surveillance. However, discrepancies between Tg levels and disease burden may occur, complicating clinical assessment. The report herein proposes to discuss such a case.

[CASE PRESENTATION] We present a case of a 15-year-old girl with a history of total thyroidectomy for PTC (age 13) who developed a new vascular neck mass. Sonographic and cytologic evidence confirmed metastasis. Serum Tg levels (evaluated using concurrent immunoassays and liquid chromatography - tandem mass spectrometry) were detectable, but not proportional to the extent of disease burden, while the fine-needle aspirate (FNA) Tg levels were markedly elevated, consistent with metastatic disease.

[DISCUSSION] This case illustrates a phenomenon in which the degree of Tg elevation in serum is not proportional to the extent of metastatic disease. We discuss the analytical and biological factors that can contribute to this discordance and highlight the importance of multimodal monitoring strategies.

[CONCLUSION] Clinicians should be aware that any detectable serum Tg in athyreotic patients indicates residual thyroid tissue or disease. However, this might not necessarily correlate well with the degree of metastatic disease. The integration of imaging, FNA cytology, and aspirate Tg measurement is essential for accurate disease assessment, particularly in cases with discordant laboratory findings.