Aims Intrathyroidal ectopic thymus (ITET) is a rare cause of paediatric thyroid nodules. Although ultrasonography of ITET demonstrates a characteristic appearance similar to that of normal thymus, accurate differentiation from other thyroid nodule etiologies by ultrasonography is difficult, and so that fine needle aspiration (FNA) is usually performed for further analysis. The aim of this study was to evaluate the utility of flow cytometry (FCM) in confirming the diagnosis of ITET in thyroid FNA samples.

Methods Five cases of ITET were retrieved from our thyroid FNA database within a 3-year period. Their clinical information, ultrasonographic features, cytology and FCM findings were retrospectively reviewed. The FCM results were compared with those of 22 T-cell acute lymphoblastic leukaemia/lymphoblastic lymphoma (T-ALL/LBL) cases.

Results The FNA smears of all five ITET cases demonstrated abundant lymphocytes of variable sizes, which included some immature lymphoid cells. No Hassall’s bodies or atypical epithelioid cells were recognised. By multicolour FCM analysis including antibodies against CD1a, CD2, CD3, CD4, CD5, CD7, CD8, CD10, CD34, TDT and TCR, all ITET cases showed antigen expression patterns consistent with normal thymocyte maturation. All T-ALL/LBL cases exhibited significant immunophenotypic aberrancy.

Conclusions The diagnosis of ITET based on FNA cytology is often inconclusive. The presence of immature lymphocytes often raises the concern for LBL. FCM with adequate antigen coverage can reliably distinguish ITET from T-ALL/LBL and make the diagnosis of ITET in FNA samples. Avoiding unnecessary further invasive procedures, providing reassurance to clinician and patient, the accurate diagnosis of ITET by FCM in FNA samples is clinically important.