Application of GATA 3 and TTF‐1 in differentiating parathyroid and thyroid nodules on cytology specimens

Background Differentiating parathyroid from thyroid lesions can be difficult on fine‐needle aspiration (FNA) due to overlapping cytomorphologic features. While the traditional parathyroid hormone (PTH) assays can help in the distinction, these tests may be cumbersome, particularly when the lesion is unexpected clinically and a needle wash is not collected at the time of FNA. Therefore, we chose to investigate the application of immunohistochemical staining (IHC) with GATA 3 and thyroid transcription factor‐1 (TTF‐1) on air‐dried cytology smears to distinguish parathyroid and thyroid lesions. Methods Air‐dried touch preparation (TP) slides were prepared from consecutively selected parathyroid and thyroid specimens. Thirteen FNA cases with the clinical concern for parathyroid lesions were also included in the study. IHC was performed on unstained and ultrafast Papanicolaou (UFP) stained air‐dried slides. Results On TP slides, GATA 3 expression was observed in all cases of parathyroid origin but no immunoreactivity was present in thyroid lesions. TTF‐1 expression was observed in all cases of thyroid origin but not in parathyroid lesions. GATA 3 and TTF‐1 expression of 13 FNA cases were consistent with the clinical impression or concurrent PTH tests. Conclusions IHC with GATA 3 and TTF‐1 on air‐dried cytology smears is a simple and effective way to differentiate parathyroid vs thyroid lesions on FNA. Air‐dried unstained and UFP‐stained slides perform equally well with IHC, but UFP‐stained slides provide the added benefit of morphologic evaluation and assessment of smear cellularity prior to IHC.