Signet-cell-simulating microfollicular adenoma of the thyroid.

A rare microfollicular adenoma of the thyroid gland with mucinous signet-cell features is described. Mucin stains demonstrate the presence of neutral mucin within some tumor cells. Immunohistochemical staining for thyroglobulin demonstrates strong thyroglobulin positivity within virtually all tumor cells, confirming the follicular-cell nature of the tumor. This tumor is contrasted with other, previously reported mucin-producing tumors of the thyroid.