Rare Case of Severe Hypercalcemia Secondary to Atypical Sestamibi Negative Parathyroid Cystic Adenoma

Primary hyperparathyroidism (PHPT) is defined as excessive secretion of parathyroid hormone (PTH) originating from the parathyroid gland. The most common cause is a single parathyroid adenoma which is typically solid. Cystic parathyroid adenomas (CPA) are the cause of about 1–2% of cases of primary hyperparathyroidism. It is known that cystic parathyroid adenomas are a result of degeneration of an existing parathyroid adenoma. SestaMIBI is an imaging study based on uptake of radioactive technetium99 and used to localize parathyroid adenomas. We describe an unusual case of severe hypercalcemia secondary to 99mTc sestaMIBI negative atypical parathyroid cystic adenoma. A 56-year-old male presented to our facility with nausea and vomiting. His past medical history included hypertension and hepatitis C with no history of fractures or kidney disease. Physical examination was normal. Upon admission the patient was afebrile with blood pressure of 170/120 mmHg and heart rate of 62 bpm. Chemistry showed Calcium of 14.5 mg/dL (8.6–10.2mg/dL), phosphorus 2.2 (2.7–4.5) mh/dL, magnesium 1.8 (1.7–2.6)mg/dL, intact PTH of 375 (15–65) pg/mL, PTH-related peptide <2.0 pmol/L(<2 pmol/L), 25-OH vitamin D of 19 ng/ml (30–80), Creatinine 1.22 (0.7–1.2)mg/dL, alkaline phosphatase 95 (40–129) units/L. He was started on aggressive hydration, calcitonin 4 units/kg, 4 mg of IV Zolendroninc acid. Neck sonogram revealed a large, complex, predominantly anechoic lesion with solid vascular components and thick internal septations in the inferior and medial aspect of the right thyroid lobe measuring 3 x 2 x 5.5 cm. Findings were confirmed with CT of the neck. Since Sestamibi scan (planar and SPECT/CT) did not show uptake in parathyroid glands, the cyst was thought to be of thyroid origin. Fine needle aspiration was not able to detect cellular material, but PTH was >100 pg/ml on the FNA sample. Otolaryngology service was consulted for parathyroidectomy. During the surgical treatment, the right upper parathyroid gland was removed with no changes in serum PTH. Next, the cystic lesion was removed with normalization of serum PTH (from 218 pg/ml to 35.2 pg/ml respectively). Intraoperative frozen section analysis was read as a cystic parathyroid adenoma. The final pathology report revealed cystic parathyroid tissue favoring parathyroid adenoma with focal atypia. Hypercalcemia resolved. Conclusions: Atypical cystic parathyroid adenomas are a rare cause of PHPT. 90% of parathyroid cysts are nonfunctional. Above mention is a case of a patient presenting with hypercalcemic crisis secondary to cystic parathyroid adenoma, which posed a diagnostic challenge as both neck ultrasound and 99mTc sestaMIBI scan were inconclusive. These findings should trigger suspicion for functional parathyroid lesions. Cystic components should be evaluated for PTH levels and if significantly elevated should be treated as a parathyroid adenoma.