ONCOCYTIC INTRATHYROID PARATHYROID ADENOMA MASQUERADING AS HURTHLE CELL NEOPLASM: FINE NEEDLE ASPIRATION CYTOLOGY OF TWO CASES

Abstract

Ectopic parathyroid adenomas in thyroid tissue are uncommon (0.7 - 6%), and their oncocytic variants are exceedingly rare. We report two cases of intrathyroid parathyroid adenoma which were diagnosed as Hurthle cell adenoma on cytology. Case 1 is a 49-year-old female with a 2.4 cm hypoechoic nodule in the left lateral neck. Electrochemiluminescent immunoassay of the aspirate revealed PTH level of 422 pg/ml, conrming the presence of hyperfunctional parathyroid tissue. Subsequent resection of the left thyroid lobe revealed an enlarged intrathyroidal parathyroid. Case 2 is a 58-year-old female with a 2.1 cm hypoechoic nodule in the posterior-mid left thyroid lobe. Strong overexpression of parathyroid hormone and Chromogranin A genes and low expression of thyrocyte-related genes suggested parathyroid origin of the cells sampled. MEN1 mutation and multiple copy number alterations indicated the neoplastic nature of the nodule. Parathyroid oxyphil cells and oncocytic thyrocytes share cytomorphological ndings and distinguishing them by cytology alone is challenging, especially when the targeted lesion is intrathyroidal. Distinguishing intrathyroid oncocytic parathyroid adenoma from oncocytic thyroid follicular lesions has signicant clinical implications as Bethesda-IV category lesions have 20%–30% risk of malignancy. While stippled chromatin or intracytoplasmic fat vacuoles may be suggestive of parathyroid origin, these are not specic. Classifying the origin of a nodule as parathyroid vs thyroid rests upon the detection of PTH in aspirate material by ECL, immunocytochemistry or next-generation sequencing. In parathyroid aspirates, PTH level 100 pg/mL is suggestive of the presence of PTH-secre ≥ ting tissue at the site biopsied or along the needle track.