We aimed to present a rare case of hyalinizing trabecular tumor that was of follicular cell origin with a trabecular growth pattern and intratrabecular hyalinization.

Thyroid fine needle aspiration biopsy was performed in our patient, a 57-year-old woman who was being followed up because of multinodular goiter. On cytological examination, the diagnosis was suspicious due to cells similar to papillary carcinoma cells. She underwent a total thyroidectomy. Macroscopically, besides numerous colloid-rich nodules with diameters ranging from 3 to 7 mm in the right thyroid, a 5 mm wide, well-circumscribed, gray-white colored, solid nodule was seen. Microscopically, the nodule was well circumscribed and had a trabecular growth pattern of elongated and polygonal cells, nuclei with grooves and cytoplasmic pseudoinclusions. Intratrabecular hyalinization was seen focally. Immunohistochemically the tumor was thyroglobulin positive, and calcitonin and HBME-1 negative. It showed strong cell membrane immunoreactivity with MIB-1. The pathological diagnosis was hyalinizing trabecular tumor with histological and immunohistochemical features.

Hyalinizing trabecular tumors that may be confused with papillary carcinoma of thyroid or medullary carcinoma usually show benign clinical behavior. The presence of PAS-positive intratrabecular hyalinization and immunohistochemical features and especially strong cell membrane positivity of MIB1 are useful features in the differential diagnosis.