Intrathyroidal Atypical Parathyroid Adenoma: A Potential Diagnostic Pittfall in Citology

  • Background / Purpose
    Atypical parathyroid adenoma (APA) is a variant of parathyroid adenoma (PA) composed of chief cells. APA shows some features of parathyroid carcinoma but lacks unequivocal invasive aspects. PA may arise in the parathyroids but also in ectopic sites, including the thyroid1.
    The aim of this study is to report a case of intrathyroidal APA misdiagnosed as thyroid nodule on fine-needle aspiration cytology (FNAC) in a 75-year-old man.

    The ultrasound (US) examination of the thyroid revealed a hypoechoic nodule of 19×15 mm. Thyroid and parathyroid function tests were normal and consistent with osteoporosis, respectively. US-guided FNAC and subsequently total thyroidectomy were performed.

    The markedly cellular smear was consistent with a Bethesda class V thyroid nodule2, showing sheets and microfollicular groups of cells with abundant eosinophilic cytoplasm, voluminous and highly pleomorphic nuclei, prominent and often multiple nucleoli. Macroscopically, the thyroid nodule was firm and well circumscribed. Histology revealed a proliferation of cells with solid-microfollicular pattern and the same cytological features observed in the smear. A thick capsule with evidence of multiple foci of capsular infiltration was detected, although unequivocal invasive features were not observed. At immunohistochemistry, cells were immunostained with chromogranin and GATA3. No immunoreactivity for thyroglobulin, TTF1, PAX8, synaptophysin and galectin-3 was found.

    Discussion & Conclusion
    The differential diagnosis between parathyroid and thyroid lesions on FNAC is difficult due to overlapping cytomorphological features and absence of specific diagnostic criteria3-5. Correlating cytomorphology and immunocytochemistry with clinical, radiological and laboratory findings is indispensable for an accurate diagnosis.


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