Melanin-producing medullary carcinoma of the thyroid gland.

An unusual case of medullary carcinoma of the thyroid gland with melanin production is described. In addition to typical histologic characteristics, some tumor cells show strong positive reaction for calcitonin by immunohistochemical study. Ultrastructurally dense-core granules and immature melanosomes are found in the same tumor cell. The tumor is thought to arise from a common precursor cell of neural crest origin with both C cell and melanocytic differentiation capable of producing both calcitonin and melanin.