Intrathyroidal parathyroid carcinoma presenting as an asymptomatic hypercalcemia

Introduction: A 26-year-old female presented with severe hypercalcemia. She was asymptomatic. Blood analysis revealed high serum calcium (13 mg/dL = 3.25 mmol/L) and elevated intact parathyroid hormone (iPTH) levels (267.5 ng/L). Results: Ultrasonography showed a mixed solid-cystic nodule of 30 mm in the left thyroid lower lobe and a nodular lesion of 14 mm posterior of the thyroid upper pole suggestive for parathyroid adenoma. Sestamibi scan and magnetic resonance imaging (MRI) revealed hyperfunctioning parathyroid tissue posterior of the left thyroid upper lobe. During surgery, a 15 mm nodule in the usual location of the left superior parathyroid was resected. Intraoperative frozen section revealed normal parathyroid tissue and intraoperative PTH levels did not decrease. Left thyroidectomy was performed due to the large solid-cystic nodule. Intraoperative PTH levels normalized 10 min later. Pathologic examination revealed a 28 mm nodule in the lower thyroid pole compatible with a parathyroid carcinoma (PC). Due to positive margins, a completion thyroidectomy was performed. 5-year follow-up showed no recurrence. Conclusions: PC is a rare entity comprising 0.5-2% of patients with primary hyperparathyroidism. Even more unusual is an intrathyroidal parathyroid gland (0.2%). Only a dozen cases of intrathyroidal PC have been reported. Our case is the second patient reported to be asymptomatic.