Background: Hyperparathyroidism is both underdiagnosed and undertreated, but the reasons for these deficiencies have not been described. The purpose of this study was to identify reasons for underdiagnosis and undertreatment of hyperparathyroidism that could be addressed by targeted interventions. Materials and Methods: We identified 3,200 patients with hypercalcemia (serum calcium >10.5 mg/dL) who had parathyroid hormone (PTH) levels evaluated at our institution from 2011 to 2016. We randomly sampled 60 patients and divided them into three groups based on their PTH levels. Two independent reviewers examined clinical notes and diagnostic data to identify reasons for delayed diagnosis or referral for treatment. Results: The mean age of the patients was 61 ± 16.5 years, 68% were women, and 55% were white. Fifty percent of patients had ≥1 elevated calcium that was missed by their primary care provider. Hypercalcemia was frequently attributed to causes other than hyperparathyroidism, including diuretics (12%), calcium supplements (12%), dehydration (5%), and renal dysfunction (3%). Even when calcium and PTH were both elevated, the diagnosis was missed or delayed in 40% of patients. For 7% of patients, a nonsurgeon stated that surgery offered no benefit; 22% of patients were offered medical treatment or observation, and 8% opted not to see a surgeon. Only 20% of patients were referred for surgical evaluation, and they waited a median of 16 months before seeing a surgeon. Conclusion: To address common causes for delayed diagnosis and treatment of hyperparathyroidism, we must improve systems for recognizing hypercalcemia and better educate patients and providers about the consequences of untreated disease. Implications for Practice: This study identified reasons why patients experience delays in workup, diagnosis, and treatment of primary hyperparathyroidism. These data provide valuable information for developing interventions that increase rates of diagnosis and referral.