The thymus as a possible source for false positivity of serum thyroglobulin: An expression study of thyroid-related genes in human thymus tissue

Objectives The thymus accumulates radioiodines in young patients with differentiated thyroid carcinoma (DTC), which could cause unnecessary suspicion of tumor recurrence and additional I-131 therapy. To examine the thymus as a possible source for false positivity of serum thyroglobulin (Tg), we investigated the expression status of thyroid-related genes in human thymus; sodium-iodide symporter (NIS), Tg, thyroid peroxidase (TPO), and thyrotropin receptor (TSHR).

Methods Among young patients (< 45 years) who had total thyroidectomy with central neck dissection for initial surgical treatment due to DTC from March to October 2010, 9 female patients (29.7 years in average) were able to acquire normal thymic tissues during central neck dissection. Reverse transcription-polymerase chain reaction (RT-PCR) was performed with primers specific to NIS, Tg, TPO, and TSHR, respectively. The relative level of the expression of each RT-PCR was estimated by comparing with GAPDH.

Results TSHR and Tg were expressed in all patients (n=9), whereas NIS and TPO were expressed in 4 patients and 7, respectively. The expression levels of TSHR and Tg were moderately or strongly positive, but those of NIS and TPO were weakly positive as compared with GAPDH. In addition, all NIS positive samples were positive for TPO at the same time.

Conclusions We demonstrated that thyroid-related genes were expressed in the normal thymus of young patients with DTC. In particular, the presence of TSHR and Tg in the thymus may provide a clue to reveal the possible source of serum Tg in young patients without residual thyroid cancer