Where Does Liquid Biopsy Add Value in Thyroid Cancer? Biological Rationale, Technological Innovation, and Clinical Utility

Thyroid cancer comprises biologically diverse entities ranging from largely indolent differentiated thyroid cancer (DTC) to aggressive poorly differentiated/anaplastic thyroid cancer and medullary thyroid cancer, generating a need for minimally invasive biomarkers that can be repeatedly sampled. This review summarizes recent advances in liquid biopsy for thyroid cancer, focusing on analytes and technologies spanning circulating tumor DNA (ctDNA)/cell-free DNA, circulating microRNAs (miRNAs), extracellular vesicles (EVs), and circulating tumor cells (CTCs). For ctDNA, we contrast qPCR/ddPCR and next-generation sequencing, tumor-informed versus tumor-agnostic strategies, the impact of low tumor fraction in DTC, clonal hematopoiesis confounding, and emerging methylation-based multi-cancer detection paradigms. For miRNAs, we highlight that bulk serum/plasma and EV-enriched compartments are not interchangeable and that regulated EV loading supports fraction-resolved biomarker development. We review recent translational EV-miRNA studies, including externally validated classifiers for metastatic disease and follicular-patterned/indeterminate nodules, and summarize the evolution of CTC research from enumeration to preoperative risk stratification and postoperative or radioiodine-related kinetics. We conclude with an indications-first framework that pairs analyte choice with clinical intent (preoperative diagnosis, initial risk stratification, response to treatment and minimal residual disease and identification of actionable alterations and resistance mechanisms) and prioritizes standardized workflows and prospective multicenter validation. Multi-analyte integration, epigenetic/fragmentomic cfDNA signals, and higher-resolution EV analytics are likely to accelerate clinical adoption, particularly in advanced thyroid cancer where circulating signal and therapeutic actionability are highest.