Triiodothyronine-secreting (toxic) adenoma of the thyroid gland: light and electron microscopic characteristics.

A patient with thyrotoxicosis due to a triiodothyronine (T3)-secreting autonomous adenoma is described. The histmorphology of the neoplasms was similar to other neoplasms previously reported. Ultrastructural features of the adenoma are compatible with a very actively secreting follicular cell and are best compared with the ultrastructure of a diffuse toxic goiter. Distinctive features that separate toxic adenomas from various thyroid carcinomas and normal thyroid parenchyma are discussed.