IntroductionDifferentiated thyroid cancer (DTC), in general, has a good prognosis with a >90% chance of being cured [1]. However, DTC can shorten life span in cases of metastatic spread. Therefore, it is important to develop tumour markers that allow early detection of tumour dissemination. Serum thyroglobulin (sTg), the currently established tumour marker for thyroid carcinoma, is organ-specific and exclusively expressed by normal as well as thyroid carcinoma cells [2], [3]. In most cases, an increase in this tumour marker indicates tumour relapse, although the reliability of laboratory tests can be limited by the presence of anti-Tg antibodies or the dependence on thyroid stimulating hormone (TSH) stimulation [3], [4]. Additional tumour markers for DTC, such as Tg-mRNA or TSH-receptor (TSH-R) exhibiting cells exist, however they are currently only used for research purposes, and clinical routine tests based on these markers are not yet available [5]. Therefore, our study investigates whether circulating epithelial cells (CEC) can be used as a direct readout of tumour dissemination into the bloodstream.CECs originate from a primary tumour, and can be detected upon release into the circulatory system. These cells are characterised by the epithelial-specific surface antigen EpCAM (Epithelial Cell Adhesion Molecule, 17A-1), which is expressed at markedly higher levels in most carcinoma cells and CECs than in normal, non-cancerous tissues [6], [7], [8], [9], [10]. EpCAM is therefore a carcinoma-associated marker, but by no means organ-specific. Various studies on different cancer types showed a correlation between the number of CECs and disease progression after treatment [6], [8], [11], [12]. This correlation is applied most frequently in assessing breast cancer prognosis [13]. Few DTC studies exist, which investigate the correlation between the presence of CECs and their change in numbers following radioiodine ablation therapy [14], [15]. However, due to the lack of organ-specific markers, the tissue origins of the CECs detected in these studies remain uncertain. This makes it necessary to further characterise the cells identified using EpCAM to evaluate the significance of CEC quantification.One approach to detect the origin of CECs is to analyse their specific messenger ribonucleic acids (mRNAs). A number of studies investigated the clinical value of analysing mRNAs of proteins typically found in the thyroid gland of DTC patients. The mRNAs analysed include Tg, TSH-R, sodium–iodide symporter (NIS) and thyroperoxidase (TPO) [2], [3], [5], [16], [17]. Although these studies analysed the total mRNA present in blood samples, they did not include analysis at the single cell level. Precise single cell level characterisation would be extremely useful in identifying the cell clones responsible for metastasis, and therefore poor prognosis. Such molecular level insights could potentially improve therapeutic decision-making, since these would allow more accurate targeting of therapeutic approaches, i.e. personalised medicine.Therefore, this study aims to determine the origin of different CECs, as defined by EpCAM expression, by testing these cells for mRNAs typically found in the thyroid gland. The priority of this study is to establish the method, and assess its technical feasibility. This study does not attempt to assess its application within the clinic or evaluate possible clinical implementation.