How low is too low? Intraoperative parathyroid hormone decline in normohormonal primary hyperparathyroidism

In normohormonal primary hyperparathyroidism, parathyroid hormone levels are normal but inappropriately elevated for the degree of hypercalcemia. The study goals were to determine intraoperative parathyroid hormone parameters predictive of (1) cure and (2) hypocalcemia in this subgroup.

We performed a retrospective cohort study comparing patients who underwent parathyroidectomy (2002-2019) for normohormonal and classic primary hyperparathyroidism. The primary outcomes were cure (calcium <10.3 mg/dL) and hypocalcemia (≤8.4 mg/dL) ≥6 months postoperatively.

In the study, 127 of 1,087 patients (11.7%) had normohormonal primary hyperparathyroidism. The groups experienced similar rates of cure (91.3% vs 94.1%, P = .23) and hypocalcemia (3.9% vs 2.9%, P = .53). However, intraoperative parathyroid hormone decline in cured patients was lower in those with normohormonal primary hyperparathyroidism (66.4% vs 84.5%, P < .0001). Receiver operating characteristic curves provided Youden's indices of 52% and 75% (cure) and 75% and 88% (hypocalcemia) for patients with normohormonal and classic primary hyperparathyroidism, respectively. Cure rates with ≥50% intraoperative parathyroid hormone decline were similar (94.1% vs 95.0%, P = .72), but hypocalcemia was more prevalent in patients with normohormonal primary hyperparathyroidism and ≥70% intraoperative parathyroid hormone decline (10.4% vs 3.3%, P = .01).

In patients with normohormonal primary hyperparathyroidism, intraoperative parathyroid hormone declines of ≥50% and ≥70% were predictive of postoperative cure and hypocalcemia, respectively. These parameters may inform intraoperative decision making and postoperative management.