Parathyroid Tissue Versus Thyroid Tissue

Abstract

A common pitfall in the diagnosis of thyroid nodules is the inadvertent sampling of parathyroid tissue, which may be difficult to distinguish from thyroid tissue on FNA due to similar cytologic features. Parathyroid cells are usually loosely clustered into small groups, often with a syncytial arrangement forming branching, loose, two-dimensional clusters. Microfollicular architecture is prevalent on ThinPrep preparations and this may contribute to overinterpretation of parathyroid tissue as follicular neoplasm of the thyroid. Parathyroid cells on ThinPrep preparations show round, centrally placed nuclei with stippled nuclear chromatin. The nuclei often appear smaller and darker on ThinPrep preparations than on corresponding FNA smears. The cytoplasm is scant to moderate. In addition to pathologic features, clinical features and radiologic appearance of the lesion are also helpful to differentiate between thyroid and parathyroid tissue. When the diagnosis is in doubt, parathyroid hormone immunohistochemical stain should be performed and positive PTH stain will confirm parathyroid origin.