It therefore seems likely that the diagnostic methods presented by Tallini and Ringel may be useful only in limited populations of patients in whom a considerable number of thyroid tumour cells expressing a high copy number of TG mRNA are circulating. In fact, our results showed no statistical difference between the samples from patients with or without distant metastasis. Further, we obtained no evidence that this method is superior to the conventional measurement of serum TG, as it was hard to utilize the TG mRNA data to differentiate the patients with metastasis from those without, whereas serum TG measurements showed more than 5 µg l -1 in all patients with distant metastasis, and the high values of interassay CV in the quantitative measurement of TG mRNA can be a disadvantage in the follow-up of a patient for a long period. Moreover, TG mRNA and serum TG measured in the same samples showed no significant correlation. These results suggest that the sources of TG mRNA and serum TG might be different, and, as discussed above, the former might not be derived from thyroid follicular cells.