Follicular adenoma of the thyroid gland with extensive bone metaplasia.

Follicular adenomas of the thyroid account for over 90% of benign neoplasms of the gland. They exhibit a wide range of morphological structures, from the classical follicular pattern to the peculiar hyalinizing trabecular pattern. Although follicular adenomas grow slowly, they are nonetheless subject to degenerative, most often hemorrhagic changes in their central portion. These hemorrhagic areas undergo further regressive changes such as sclerosis and calcification. However, the detection of a true bone formation with a trabecular structure and the presence of marrow is a very rare occurrence. A follicular adenoma with central cartilaginous metaplasia has been reported in literature but, to our knowledge, a follicular adenoma with bone metaplasia has never been described.