Thyroglobulin mRNA quantification in the peripheral blood is not a reliable marker for the follow-up of patients with differentiated thyroid cancer

Background: The detection of serum thyroglobulin (Tg) by immunoassay is widely used to detect residual, recurring or metastatic thyroid carcinoma tissue in patients with differentiated thyroid cancer (DTC) after total thyroidectomy and radioiodine therapy. However, this method requires thyroid hormone withdrawal to increase sensitivity and is limited by the interference of anti-Tg antibodies. To solve these problems, the detection of Tg mRNA from circulating thyroid cells by reverse transcription (RT)-PCR has been suggested as an alternative method. However, different previous reports show discrepant conclusions as to the clinical usefulness of Tg mRNA quantification. Methods: We compared three methods of blood collection and RNA extraction, and quantified Tg mRNA (by real time RT-PCR) in the peripheral blood of a) probands without thyroid disease (n = 42), patients with b) thyroid autonomy (n = 15), c) Graves' disease (n = 22), d) euthyroid goiter (n = 6), and in DTC-patients after thyroidectomy and radioiodine therapy e) with (n = 16) and 0 without (n = 3 7) metastasis. As the use of citrate blood in combination with a subsequent separation of mononuclear cells showed a significantly better RNA yield than the extraction of RNA from EDTA or citrate blood without the separation of mononuclear cells, this was the method used. Total RNA was reverse transcribed with random hexamer primers and Tg mRNA was amplified by real time RT-PCR using specific primers and hybridization probes. The Tg mRNA concentrations were normalized to beta-actin mRNA concentrations. Results: Mean circulating Tg mRNA for each group detailed above, expressed as the ratio of Tg to beta-actin concentrations X 1000, were: a) 2.3 (range 0.03-70.89), b) 0.25 (range 0.02-0.55), c) 0.31 (range 0.05-1.36), d) 0.18 (range 0.08-0.35), e) 0.57 (range 0.03-3.03) and 0 0.17 (range 0.02-0.60). Furthermore, we found no correlation between serum Tg and Tg mRNA. Conclusions: In summary, our data do not show significant differences in Tg mRNA expression between the investigated groups. Therefore, the detection and quantification of Tg mRNA in peripheral blood is unlikely to be suitable for the follow-up of DTC.