MEN1 and primary hyperparathyroidism: We conducted a retrospective cohort study using the Korean National Health Insurance Service database (2003–2020), identifying clinically suspected MEN1 cases via two operational definitions: (1) ICD-10 MEN1 code (D44.8) with medical service records and (2) diagnoses or interventions for at least two MEN1-associated conditions (primary hyperparathyroidism, pituitary adenoma, or duodenopancreatic neuroendocrine tumors).