Hyalinizing trabecular adenoma of the thyroid. A report of three cases with immunohistochemical and ultrastructural studies.

We have studied three examples of benign thyroid tumours which conform to the lesion recently described as hyalinizing trabecular adenoma. The prominent feature of this trabecular epithelial lesion is the extracellular deposition of an eosinophilic material that does not show the features of amyloid; the epithelial cells may be elongated, sometimes radially arranged around the eosinophilic material. All three tumours showed positive immunocytochemical staining for thyroglobulin, keratin, chromogranin and neuron-specific enolase. One tumour showed isolated cells with immunoreactivity for somatostatin, argyrophil cytoplasmic granularity with the Grimelius technique, and ultrastructurally demonstrable cytoplasmic electron-dense endocrine granules. The hyaline extracellular material in all three tumours showed strong immunoreactivity for both type IV collagen and laminin. Previous radiation may be important in the causation of this tumour, and it shows a frequent association with severe thyroiditis. We conclude that the main histological feature of this lesion, the hyaline material, is due to the overproduction of a basement membrane-like material by the neoplastic follicular cells. The presence in one tumour of evidence of endocrine differentiation may reflect the potential for follicular cells to show biphasic differentiation, a feature which may be more frequent than has been previously recognized. Several of the other features of this tumour--for example the frequent intranuclear cytoplasmic inclusions--may lead to diagnostic problems, and the entity deserves wider recognition.