RESUMEN
BACKGROUND: The cytologic features of C-cell hyperplasia of the thyroid have not been previously addressed in the literature. We describe the first case, to our knowledge, of C-cell hyperplasia that was suggested by fine needle aspiration. CASE: Cellular material was obtained from a nonnodular region of the thyroid gland in a 67-year-old male with chronic diarrhea, unexplained elevated serum calcitonin, no clinically detectable thyroid mass and no known medical or family history of an endocrine disorder. Aspiration yielded a scant bimodal cell population composed of benign follicular cells and a second population of larger cells, later confirmed as C-cells via immunohistochemistry. Although the diagnosis of medullary carcinoma was entertained, the absence of a discrete mass clinically and the presence of two interspersed, distinct cell populations suggested the alternate diagnosis, C-cell hyperplasia, which was confirmed by subsequent thyroidectomy. CONCLUSION: C-cell hyperplasia can mimic medullary carcinoma biochemically, and this case suggests the possible role of fine needle aspiration of the thyroid to distinguish between the two. In patients with elevated serum calcitonin and absence of a discrete thyroid nodule, the finding of clusters of calcitonin-positive cells intermixed with normal follicular cells by fine needle aspiration may provide a means of making a presurgical diagnosis of C-cell hyperplasia.