Disseminated calcitonin-poor medullary thyroid carcinoma in a patient with calcitonin-rich primary tumor.

A multiple endocrine neoplasia type Ia patient who initially had a calcitonin-rich medullary thyroid carcinoma followed an indolent clinical course for 7 years after surgical resection. Sudden rapid dissemination of tumor led to death 2 years later. At autopsy, calcitonin immunostaining of tumor was diffusely negative, with slight positivity in less than 5% of cells. The loss of a marker of differentiation in association with the marked change in the clinical course suggests the development within the well-differentiated tumor of a poorly differentiated, biologically aggressive population of tumor cells.