Primary Hyperparathyroidism With Undetectable Intact Parathyroid Hormone.

Hypercalcemia can result from either hyperparathyroidism or non-parathyroid conditions. When hypercalcemia is accompanied by undetectable parathyroid hormone (PTH) levels, hyperparathyroidism is rarely considered the diagnosis. Herein, we report the case of a 65-year-old Caucasian woman referred to our hospital for further evaluation of hypercalcemia. Her symptoms included fatigue and brain fog, with undetectable PTH levels. A comprehensive workup, including a series of laboratory and imaging tests, excluded common non-parathyroid causes such as malignancy and familial hypocalciuric hypercalcemia. Ultrasound identified a likely enlarged parathyroid gland, which was further confirmed by a sestamibi scan. After 2 weeks of cinacalcet treatment, the patient's calcium levels decreased, indicating the parathyroid gland as the likely source of hypercalcemia. Parathyroidectomy was subsequently performed, revealing a 1927 mg adenoma. Postoperatively, the patient's calcium levels normalized, PTH levels became detectable within the normal range, and her symptoms resolved, with a marked improvement in energy. This case demonstrates that primary hyperparathyroidism can present with hypercalcemia and undetectable PTH. A genetic mutation in the PTH gene within the adenoma may explain the undetectable PTH levels preoperatively.