Mediastinal Intrathymic Parathyroid Adenoma: A Case Report and Review of the Literature

Abstract

The classic clinical vignette of primary hyperparathyroidism is well described as "bones, stones, abdominal moans, and psychiatric overtones" to reflect the effects of excess parathyroid hormone (PTH) and calcium. Most commonly, primary hyperparathyroidism is due to a functional parathyroid adenoma situated by the thyroid gland. Rarely, the primary focus of autonomously produced PTH is located ectopically within the mediastinum. A 19-year-old Caucasian female with no relevant past medical history presented to the emergency department with tachycardia, nausea, vomiting, and a five-day history of vague, mid-abdominal pain. Initial computed tomography (CT) with contrast of the abdomen and pelvis was negative for acute findings, and she subsequently underwent biochemical screening. The patient was found to have elevated serum calcium and PTH, raising suspicion for the diagnosis of primary hyperparathyroidism. Further evaluation for a parathyroid adenoma was negative by a CT scan of the neck and thyroid ultrasound. A nuclear medicine parathyroid single-photon emission computed tomography (SPECT)/CT with technetium (Tc) 99m sestamibi found an abnormal nodular uptake within the left prevascular mediastinum suggestive of an ectopic parathyroid adenoma. A left-sided, video-assisted thoracoscopic surgery (VATS) with successful excision of the ectopic mediastinal parathyroid adenoma was performed. Surgical pathology revealed that the parathyroid adenoma was completely excised and surrounded by thymus and adipose tissue. The patient tolerated the procedure well and was discharged without further complications. The rarity of mediastinal, intrathymic parathyroid adenomas resulted in delayed diagnosis in this patient, understandably so as errant embryogenesis does not occur commonly. Visualization with SPECT/CT and successful specimen excision by minimally invasive VATS resulted in the accurate diagnosis and ultimate cure of this patient's primary hyperparathyroidism.

Keywords: ectopic parathyroid tissue; hypercalcemia; hyperparathyroid; intra-thymic; solitary parathyroid adenoma; spect-ct; video-assisted thoracoscopic surgery (vats).

Figure 1. Parathyroid and thyroid scans

Representative nuclear medicine studies indicating a lack of avidity in the peri-thyroid region (panel A) with a concomitant increase in metabolic activity around the prevascular mediastinal area (red arrows, panels B-D) overlayed on non-contrast CT imaging in coronal (panel B), sagittal (panel C), and axial (panel D) planes. Following intravenous (IV) administration of technetium-99m sestamibi, the thyroid (panel A) and mediastinum (panels B-D) were evaluated for increased uptake. The thyroid, demarcated by black (low activity) to white (high activity) shading (panel A), showed uptake within normal ranges. However, the mediastinum, demarcated by red (low activity) to yellow (high activity) shading, showed an unusual focus of increased uptake in the prevascular mediastinal area (red arrows, panels B-D).

Figure 2. Confirmatory CT imaging of prevascular mediastinal parathyroid adenoma

High-quality CT scan without contrast of the chest with isolation (yellow arrows) and orthogonal measurements based on the longest primary axis of the ovoid 17 mm x 11 mm mediastinal lesion (yellow-colored font adjacent to white-dashed line).

Figure 3. Intraoperative video-assisted thoracoscopic surgery (VATS) demonstrating excision of the causative lesion

A rounded 1 cm mass was seen in the subpleural space, located below the aortic arch above the heart, and anterior to the phrenic nerve by several millimeters (red arrow, panel A). Electrocautery was lightly applied to the pleura, opening it, and dissection was very carefully performed with thoracic graspers. Gentle elevation with the suction tip was able to remove the lesion with minimal bleeding (panels B and C).

Figure 4. Microscopic surgical pathology of the excised lesion

Low-magnification (40x) microphotograph demonstrating the parathyroid adenoma (lower part), which is located in the thymus (mid-upper part). Hematoxylin and eosin stain of the 2.4 cm x 1.5 cm x 0.5 cm surgically excised specimen, representing an intrathymic parathyroid adenoma. Microscopically, most of the parathyroid gland is surrounded by a capsule; however, in this focal area, the parathyroid glandular tissue (P) is mixed with thymus tissue (T). The thymus tissue contains Hassall corpuscles (H), and there are interspersed areas of adipose tissue (A). There is no evidence of increased mitosis, and these features are not typical of invasion; therefore, this represents an ectopic parathyroid gland within the thymus, which developed into a parathyroid adenoma.