A 41-year-old woman had a 1-to 2-cm asymptomatic thyroid mass, which had been monitored by an endocrinologist for several years with serial sonography. Noting that it had enlarged in the face of normal thyroid function tests, the endocrinologist ordered a fine-needle aspiration biopsy, which showed a cellular follicular lesion. Although some of the nuclear changes seen in papillary carcinoma were present, the cells did not fulfill all of the cytologic criteria for that diagnosis. The patient was referred to Baylor University Medical Center for surgery; she brought with her two ultrasound images (Figure (Figure11) and a chest x-ray that showed a minimal leftward deviation of the trachea. Figure 1 The two preoperative thyroid ultrasound images showing a well-cir-cumscribed, solid lesion with heterogeneous echotexture. The mass was 2.4 cm (longitudinal) × 1.9 cm (anteroposterior) × 1.9 cm (transverse). A thyroidectomy was performed in the patient, and 15 lymph nodes in her neck were dissected from the central compartment of the neck. The excised mass was like a small rock (Figure (Figure22). The patient's surgery was uneventful, and she was discharged home. She had no postoperative complications and has been started on a replacement dose of levothyroxine. She has resumed all normal activities. Figure 2 Gross features of a hyalinizing trabecular tumor of the thyroid. It has a delicately lobulated cut surface, with a yellow cast and gaping vessels. Pathological evaluation showed a 2.0 cm encapsulated nodule in the right lobe with a distinctly trabecular pattern, elongated tumor cells with enlarged nuclei with longitudinal grooves, occasional intranuclear cytoplasmic intrusions, and eosinophilic hyalin stroma. The features were strongly in favor of a hyalinizing trabecular tumor (HTT). There was advanced lymphocytic thyroiditis in the nonneoplastic right lobe parenchyma and in the left lobe (Figure (Figure33). The resected lymph nodes revealed no metastatic disease. The differential diagnosis of HTT is chiefly medullary thyroid carcinoma and papillary thyroid carcinoma, since several histologic features are shared with these two entities. Immunohistochemical studies were positive for thyroglobulin and thyroid transcription factor-1 and negative for calcitonin, confirming the follicular cell origin of the tumor and excluding medullary thyroid carcinoma. An immunostain for the tumor proliferation marker MIB-1 demonstrated a distinctive cell membrane pattern that has been described in HTT but not in papillary carcinoma (Figure (Figure44). A test for the mesothelial cell surface protein recognized by monoclonal antibody HBME-1 was invalid, as it demonstrated patchy staining of the tumor and the nonneoplastic thyroid tissue in this case. Cytokeratin 19 was negative in the tumor cells. Figure 3 Hyalinizing trabecular tumor (“adenoma”). (a) A distinctive trabecular pattern is seen from low power in this encapsulated tumor. (b) Eosinophilic hyaline material is seen surrounding the tumor cells in many areas. (c) The nuclei are enlarged ... Figure 4 MIB-1 staining in the specimen demonstrates a very low proliferative index; the rare positive nuclei in this field are endothelial cell nuclei—no tumor cells are staining. DIAGNOSIS: Hyalinizing trabecular tumor.