Hyperparathyroidism is a rare entity in pediatrics. It is defined as the increased production of parathyroid hormone. It may be due to a primary defect of the parathyroid glands (primary hyperparathyroidism) or to a compensatory parathyroid hormone production to correct hypocalcemia states of various origins (secondary hyperparathyroidism). We describe the case of a 15-year-old adolescent girl with a history of several months of deteriorating school performance, anxiety crises, weight loss, and tachycardia. Laboratory results showed hypercalcemia, hypophosphatemia, increased parathyroid hormones, and hypovitaminosis D; imaging studies showed generalized bone involvement and parathyroid nodular formation. A parathyroidectomy and right hemithyroidectomy were performed, after which she presented persistent hungry bone syndrome. The anatomopathological diagnosis was an atypical parathyroid tumor. We describe the form of presentation, the results of the different complementary tests performed, and the short- and long-term evolution.