Incidental medullary thyroid carcinoma in sporadic hyperparathyroidism. An expansion of the concept of C-cell hyperplasia.

Two patients with sporadic primary hyperparathyroidism were found to harbor tiny medullary thyroid carcinomas. In addition, parafollicular-cell hyperplasia was recognized in one of these thyroids by the use of immunoperoxidase stain to localize calcitonin. The possible relationships between these lesions are discussed.