Intrathyroidal parathyroid carcinoma: report of an unusual case and review of the literature

Abstract

Intrathyroidal parathyroid carcinoma is an exceedingly rare cause of primary hyperparathyroidism. A 51-year-old African American female presented with goiter, hyperparathyroidism, and symptomatic hypercalcemia. Sestamibi scan revealed diffuse activity within an enlarged thyroid gland with uptake in the right thyroid lobe suggestive of hyperfunctioning parathyroid tissue. The patient underwent thyroidectomy and parathyroidectomy. At exploration, a 2.0 cm nodule in the usual location of the right inferior parathyroid was sent for intraoperative frozen consultation, which revealed only ectopic thyroid tissue. No parathyroid glands were identified grossly on the external aspect of the thyroid. Interestingly, postoperative parathyroid hormone levels normalized after removal of the thyroid gland. Examination of the thyroidectomy specimen revealed a 1.4 cm parathyroid nodule located within the parenchyma of the right superior thyroid, with capsular and vascular invasion and local infiltration into surrounding thyroid tissue. We present only the eighth reported case of intrathyroidal parathyroid carcinoma and review the literature.

Figure 1

Intraoperative monitoring of PTH levels. Graph depicts marked decrease in parathyroid hormone levels following thyroidectomy. PTH hormone dropped initially from 579 to 40 pg/mL, then 83 pg/mL, and finally to 16 pg/mL. (PTH level is on y-axis, time is on x-axis, and shaded box represents PTH reference range from 15 to 65 pg/mL.)

Figure 2

Microscopic appearance of intrathyroidal parathyroid carcinoma. (a) Extracapsular extension (“mushroom projection”) of neoplastic cells through the capsule into surrounding thyroid parenchyma (hematoxylin and eosin stain (H&E); magnification 20x). (b) Immunohistochemistry for parathyroid hormone (PTH) shows strong and diffuse immunoreactivity in tumor cells and highlights areas of capsular invasion. There is nonspecific staining of colloid in thyroid follicles in the upper left (20x). (c) Foci of vascular invasion seen at the periphery of the tumor (H&E; 100x). (d) Trabecular arrangement of tumor cells traversed by thick bands of collagen (H&E; 40x). (e) Focal perivascular arrangement of tumor cells around vessels (H&E; 200x). (f) Cytologic features of tumor cells demonstrating tumor cell monotony with clear to eosinophilic cytoplasm and prominent macronucleoli (H&E; 400x).