Neonatal hypocalcemia is an uncommon but potentially serious condition that may be the first sign of an undiagnosed maternal disorder. Among the rarer causes is maternal primary hyperparathyroidism, which can suppress fetal parathyroid function through chronic intrauterine calcium exposure, leading to transient neonatal hypoparathyroidism and hypocalcemia. This case describes how neonatal seizures prompted the diagnosis of asymptomatic maternal hyperparathyroidism caused by a parathyroid adenoma.

A 37-year-old woman delivered a term female infant on August 22, 2022. The neonate developed seizures shortly after birth while staying at a postpartum care center. Laboratory evaluation showed hypocalcemia (6.2 mg/dL, reference range: 8.8 - 10.8 mg/dL) and low parathyroid hormone levels (13.08 pg/mL, reference range: < 64.87 pg/mL). Genetic exams were performed and inherited endocrine disorders were excluded. Given these findings, the mother was referred to endocrinology clinic for further evaluation.

The mother was asymptomatic but had a history of nephrolithiasis. Laboratory studies revealed hypercalcemia (11.7 mg/dL, reference range: 8.9 - 10.3 mg/dL), hypophosphatemia (2.4 mg/dL, reference range: 2.7 - 4.5 mg/dL), and elevated intact parathyroid hormone (199.21 pg/mL, reference range: < 64.87 pg/mL). Renal and liver function tests were normal. A 24-hour urine calcium test confirmed hypercalciuria (386.4 mg/dL, reference range: 100-300 mg/day). Imaging with ultrasound and parathyroid scan identified a right superior parathyroid lesion. Testing for type 1 multiple endocrine neoplasia was negative, and a Bone densitomertry showed normal bone mineral density. The patient was referred to surgical department under the tentative diagnosis of primary hyperparathyroidism secondary to a parathyroid adenoma. She underwent elective right superior parathyroidectomy on February 1, 2023. Intraoperatively, a 1034 mg parathyroid adenoma was removed. The procedure was uncomplicated, and the patient recovered well. Pathology confirmed the diagnosis, showing a well-circumscribed chief cell adenoma without atypia or necrosis. Immunohistochemistry was positive for PTH and cytokeratin, with a Ki-67 index of approximately 2%. After surgery, the mother's serum calcium normalized.

This case highlights the importance of evaluating maternal hyperparathyroidism in neonates with unexplained hypocalcemia. Maternal primary hyperparathyroidism can be asymptomatic but has significant consequences for both the mother and fetus. Early diagnosis and surgical treatment are essential to prevent long-term complications. Neonatal hypocalcemia may be the first and only clue, emphasizing the value of thorough maternal assessment in such cases.