A case of hyalinizing trabecular adenoma of the thyroid

A case of hyalinizing trabecular adenoma of the thyroid in a 38-year-old woman is reported. Cytologically, intranuclear pseudoinclusions and grooving of the nuclear membrane were often found, mimicking papillary carcinoma. On the other hand, some cytologic features, such as poor cohesion of short spindle cells and amyloid-like material, were suggestive of medullary carcinoma. The resected specimen showed the typical histological pattern of hyalinizing trabecular adenoma. Tumor cells were arranged in trabecular cords and hyaline materials. The cells were elongated, and some nuclei were grooved and showed intranuclear pseudoinclusions. Hyalinizing trabecular adenoma should be considered when an unusual combination of the features of both papillary and medullary carcinoma is observed.