An Atypical Clinical Presentation of a Parathyroid Adenoma

Introduction:
Parathyroid adenomas usually appear as well-defined, homogeneous, hypoechoic masses on ultrasound (US). However, atypical features on US including heterogeneity, cystic changes, and calcifications raise suspicion of malignancy. We present a patient with a suspicious atypical clinical presentation of a parathyroid adenoma.

Case Description:
A 29-year-old female presented after a fall with a new right non-displaced proximal tibial pathologic fracture through a large lytic bone lesion. Prior to bone biopsy and operative fixation of the fracture, lab work was significant for a calcium level of 14.8 mg/dL, albumin 4 g/dL, intact parathyroid hormone (PTH) 1,176 pg/mL, 25-hydroxy vitamin D 14 ng/mL, magnesium 1.6 mg/dL, alkaline phosphatase level 483 U/L and phosphorus 2.2 mg/dL. She denied any personal or family history of hypercalcemia, nephrolithiasis, osteoporosis, excessive calcium or vitamin D intake, malignancy or symptoms of abdominal pain, nausea, vomiting, constipation, or dysuria. Of note, the patient had a prior biopsy of the left lytic hip bone lesion eight months prior. Pathology initially revealed a benign process concerning for hemangioma of the bone or fibrous dysplasia; however, retrospective review was consistent with osteitis fibrosa cystica.

A thyroid US revealed a heterogeneous 3.9 x 2.4 x 3.0 cm complex solid and cystic mass in the neck posterior to the thyroid between the trachea and carotid, concerning for a malignant neoplasm. Parathyroid scan revealed an intense nodular focus of radiotracer uptake at the inferior pole of the right thyroid lobe suspicious for parathyroid neoplasm. She required intravenous (IV) normal saline, calcitonin, and IV pamidronate due to persistent hypercalcemia. She underwent a right hemithyroidectomy and inferior parathyroidectomy with a closed treatment of her tibial fracture. A pre-excision intraoperative PTH level was 1,473 with a post-excision PTH level of 114. Pathology revealed a cystic parathyroid adenoma. Hungry bone syndrome was prevented postoperatively with calcium carbonate 1 g three times daily, calcitriol 0.5 mg twice daily, and vitamin D3 50,000 IU weekly.

Discussion:
This atypical presentation of a parathyroid adenoma is significant because it highlights similarities it can share with parathyroid carcinomas. Initial imaging was highly suspicious for features of malignancy, but surgical pathology was consistent with a parathyroid adenoma. Furthermore, this case is unique because the pathologic finding of osteitis cystica is unusual in the early biochemical diagnosis of hyperparathyroidism. Therefore, clinicians should be aware of this form of disease and include it in their differential in a patient with multiple bone cysts.