Answer: Parathyroid adenoma

Histology: The resection specimen consists of a 2 cm. tan oval nodule. By light microscopy, it is comprised of follicles filled pink colloid-like material. The follicles are back-to-back with sparse intervening stroma. In particular, stromal adipose tissue is not present. The epithelium lining the follicles have round uniform nuclei with an evenly dispersed chromatin pattern. The nuclei are not optically clear, crowded or overlapping. Lymphoid tissue is not observed.

Discussion: Although the error rate in identifying parathyroid tissue is low, histologic recognition of parathyroid tissue is not always straightforward. In particular, there are instances where parathyroid adenomas and cellular thyroid nodules are virtually indistinguishable by routine light microscopy. Three light-microscopic features are especially prone to misinterpretation:
(1) prominent follicle formations in a cellular parathyroid gland, simulating a cellular thyroid nodule;
(2) oncocyte-rich nodules in a cellular parathyroid gland, simulating a Hurthle cell lesion of the thyroid; and
(3) the appearance of fat in a thyroid nodule, due to edema or adipose metaplasia, simulating the stromal compartment of the parathyroid gland.
When any one of these features is well developed, confirmation of tissue type may have to await immunohistochemical confirmation for evidence of thyroid follicular differentiation (e.g. thyroglobulin immunoreactivity) and chief cell differentiation (e.g. parathyroid hormone immunoreactivity). In the present case of a parathyroid adenoma, the conspicuous formation of follicles with colloid-like material led to an erroneous frozen section diagnosis of a benign extrathyroidal thyroid nodule. The ultimate diagnosis of parathyroid adenoma required immunohistochemistry. The cells were immunoreactive for parathyroid hormone but not for thyroglobulin.