Intrathyroidal parathyroid cancer presenting as a left neck mass.

Parathyroid carcinoma is a rare clinical entity with a reported incidence of between 0.5% and 4% of all patients with primary hyperparathyroidism. Equally as unusual is the presence of an intrathyroidal parathyroid gland, which is usually found at the time of exploration by exclusion after extensive dissection has been performed in the usual anatomic locations for the parathyroid glands. This is the description of a case of intrathyroidal parathyroid cancer that presented as primary hyperparathyroidism with a left neck mass. On exploration, an enlarged left lobe of the thyroid was present and no left superior pole parathyroid gland was found with exploration. A left thyroid lobectomy was performed that confirmed a parathyroid cancer that was entirely intrathyroidal. This is the first known reported case in the literature of a carcinoma arising within the thyroid gland.