The vast majority of thyroid carcinomas are differentiated tumours that originate from the follicular epithelium and show Total RNA from a normal thyroid was added to that from 10 ml of blood from a healthy subject, after which real-time quantitative RT-PCR, as described in Methods was carried out. The copy number of GAPDH mRNA was simultaneously measured, and the expression levels of TG mRNA were calculated as the ratio of TG and GAPDH mRNA. The results are shown with mean ±SD for triplicate determinations either follicular or papillary structure. Serum TG immunodetection measurements are currently the best available indicator of tumour metastases, even though their diagnostic usefulness has several limitations. Circulating tumour cells can be detected by sensitive PCR amplification of tumour-specific abnormal gene sequences or through the detection of tissue-specific abnormal gene transcripts normally absent in the peripheral blood. Thyroid tumours represent the ideal target for such studies because they actively express tissue-specific markers such as TG and TPO. Despite promising reports in 1998 on the detection of TG mRNA in peripheral blood, our preliminary study showed that TG transcripts were detectable in all samples tested, including the ones from healthy subjects. Recently, 2 studies suggesting limitations in the clinical applications of this method have been reported. Wingo et al have detected TG transcripts in the peripheral blood of healthy subjects that can be measured by real-time quantitative RT-PCR (Wingo et al, 1999) . Bojunga et al have found that TG mRNA expression is not specific to thyroid tissue and is not correlated with a diagnosis of thyroid cancer in patients, and they have recommended further examination of this method by quantitative measurement (Bojunga et al, 2000) . We therefore decided to re-evaluate this method using samples obtained from patients after total thyroidectomy, in which TG transcripts in the circulating thyroid cells from the normal thyroid may be negligible. We detected TG transcripts in all the samples tested, even in that of the patient diagnosed with medullary carcinomas, in whom the circulating of thyroid follicular cells was not likely to occur. Real-time quantitative RT-PCR assay confirmed the expression of TG mRNA in these samples, in which an increase in the fluorescent signal from hybridized TG cDNA-specific probe was observed.