Metastases to the thyroid gland from uncommon primaries: a case series

Metastasis to the thyroid gland is an uncommon clinical finding that is often mistaken for primary thyroid carcinoma because of overlapping radiologic and cytologic features. We report three patients with thyroid metastases who underwent clinical evaluation, ultrasound, fluorodeoxyglucose positron emission-computed tomography, fine-needle aspiration cytology, and confirmatory histopathology with immunohistochemistry (IHC) to establish the primary source. All three patients presented with thyroid nodules classified as Bethesda VI on cytology with cellular features pointing toward a nonthyroid origin. Cell block preparation and, in selected cases, IHC were essential for distinguishing metastases to the thyroid gland from a primary thyroid carcinoma. The primary tumors identified in this series were a gastric adenocarcinoma with signet ring morphology, a choriocarcinoma, and a leiomyosarcoma. These cases highlight that, although rare, thyroid metastases should be considered in the differential diagnosis of atypical thyroid nodules. In cases where sonographic findings resemble those of primary thyroid cancer, it is essential to maintain a high index of suspicion in patients with suggestive cytological features. Accurate diagnosis relies on a combination of IHC, comprehensive whole-body imaging, and targeted biopsy of the suspected primary site.