Thyroid thymoma in childhood.

A nontoxic thyroid nodule was detected in a female infant soon after birth. At 4 years of age, the nodule was removed. Histological examination disclosed a Iobulated spindle-cell neoplasm with focal epithelioid differentiation. The tumor demonstrated diffuse immunopositivity for low-molecular-weight keratins with scattered cells staining for S- 100 protein or leukocyte common antigen; the tumor was negative for calcitonin and thyroglobulin. By electron microscopy, the spindle-shaped cells contained bundles of intracytoplasmic tonofilaments and desmosomes. The light- and electron-microscopical features and immunohistochemical profile of this tumor were those of the recently described primary thyroid thymoma. Thymoma should be considered as a possible, albeit rare, cause of a thyroid mass in infancy or childhood.