Metastatic Hobnail Papillary Thyroid Carcinoma Initially Diagnosed by Pleural Effusion: Case Report of a Rare and Intriguing Entity.

Hobnail papillary thyroid carcinoma (HNPTC) is an unusual and aggressive variant of PTC. Malignant pleural effusion secondary to thyroid carcinoma is a rare event reported in less than 1% of cases. Herein we present a case of metastatic HNPTC initially diagnosed by pleural effusion cytology, with a very poor outcome. A 42-year-old male with no previous cancer history, presented with an anterior neck mass growing for the past year. A computed tomography (CT) scan revealed a large complex cystic lesion with mass effect on the anterior thyroid gland, along with multiple neck and mediastinal necrotic lymph nodes, suspicion for metastasis. CT of the thorax showed lung nodules and a large left pleural effusion. Fluid was drained and sent for cytologic examination which revealed malignant cells predominantly in a micropapillary pattern with apically placed, bulged, "hobnail-like" appearance; intranuclear pseudo-inclusions or chromatin clearing was not seen. Based on cytomorphologic findings and immunohistochemical results (CK7 and PAX-8 positive; TTF-1, Napsin-A, and mesothelial markers negative), final diagnosis of HNPTC metastatic to pleural fluid was made. Fine-needle aspiration of neck mass showed similar cytomorphologic findings, confirming a thyroid origin. However, the patient experienced complications, rapid deterioration and died 6 months after diagnosis. The intersection of clinical-radiologic findings, cytology, and immunohistochemistry guided the identification of this rare variant of PTC as the origin of the malignant pleural effusion. We believe this case provides a valuable insight into the complexities involved in the diagnosis of thyroid carcinoma and emphasizes the significance of proper identification of rare variants.