Answer: C cell hyperplasia

Histology: This case was a prophylactic thyroidectomy performed because of a history of medullary carcinoma within the family. Towards the upper third portion of both lobes, C cell hyperplasia was evident. This is characterized by a proliferation of cells with uniform round nuclei and abundant amphophilic cytoplasm located within some of the follicles, although in some of the planes of sections it is difficult to appreciate that the hyperplastic cells are part of a follicle. More than six C cells per follicle is diagnostic of C cell hyperplasia. These cells strongly express calcitonin immunohistochemically.

Discussion: Solid cell nests which are thought to represent remnants of the ultimobranchial body are small nests composed of oval cells with occasional clear cells. There may be occasional small glandular lumina with mucinous secretion. These lesions lack the relationship to follicles seen with C cell hyperplasia and do not express calcitonin. They also are not preferentially located towards the upper third of the thyroid. It should be recognized that one may see an increased number of C cells around solid cell nests. Medullary carcinomas may have similar cytologic features, yet would be invasive and not restricted to follicles. Papillary microcarcinoma would have the characteristic nuclear features of papillary carcinoma. These would consist of more ovoid, overlapping, irregular nuclei often showing optical clearing, nuclear grooves, and nuclear pseudoinclusions. Papillary microcarcinomas often are associated with sclerosis.