Ectopic PTH-producing parathyroid cyst inside the thymus: a case report.

Area of Science:

• Endocrinology
• Surgical Pathology
• Medical Imaging

Background:

• Hyperparathyroidism is characterized by elevated parathyroid hormone (PTH) leading to hypercalcemia.
• While PTH-related protein causes hypercalcemia in malignancy, primary PTH-producing tumors are rare.
• Ectopic parathyroid tissue and glands are difficult to diagnose due to embryological complexity and lack of clear histopathological criteria.

Observation:

• A 57-year-old woman presented with hypercalcemia and elevated PTH levels, but no enlarged parathyroid glands.
• 99mTc-MIBI indicated a localized, slightly hyperfunctioning parathyroid tissue in the anterior mediastinum, atypical for parathyroid tissue.
• Venous sampling confirmed ectopic PTH production, leading to the diagnosis of a cyst-like tumor in the anterosuperior mediastinum.

Findings:

• Histopathological examination revealed an ectopic parathyroid cyst within the thymus.
• The cyst fluid contained extremely high intact-PTH levels (19,960,000 pg/mL).
• Resection of the mediastinal thymus and ectopic parathyroid cyst successfully decreased calcium and intact-PTH levels.

Implications:

• This case highlights the importance of considering ectopic parathyroid cysts within the thymus in the superior mediastinum.
• Careful diagnosis, considering both positive and negative 99mTc-MIBI findings, is crucial for superior mediastinal cysts.
• Histological examination is key for confirming ectopic PTH-producing parathyroid glands, especially when cyst fluid demonstrates significantly elevated intact-PTH levels, leading to hyperparathyroidism and hypercalcemia.