Incidental Ultimobranchial Body Remnant (Solid Cell Nest) in a Multinodular Goiter

Solid cell nests are vestigial remnants of the ultimobranchial body (UBB) and are infrequently identified in routine thyroid specimens. Due to their solid epithelial architecture, they can closely resemble malignant thyroid tumors, making accurate recognition crucial. We report the case of a 46-year-old woman who underwent total thyroidectomy for a benign multinodular goiter. Histopathology revealed nodular hyperplasia along with an incidental microscopic focus of epithelial cells arranged in solid nests and cords. These cells exhibited polygonal morphology with eosinophilic cytoplasm and bland nuclei, without atypia or mitotic activity. Immunohistochemistry showed positivity for cytokeratin (AE1/AE3) and p63 and negativity for thyroglobulin, thyroid transcription factor-1, and calcitonin, consistent with a UBB remnant (solid cell nest). This case highlights the importance of recognizing solid cell nests as benign embryologic derivatives that may mimic neoplastic processes. Proper identification, supported by an appropriate immunohistochemical panel, is crucial to prevent diagnostic errors and unnecessary clinical concern in routine thyroid pathology.