Recently, primary hyperparathyroidism, which represents a medical and socially significant problem, has become increasingly relevant. The high prevalence of forms without pronounced clinical symptoms, as well as the low awareness of doctors regarding this pathology, lead to late diagnosis, delayed initiation of treatment and, as a consequence, the development of serious multisystem complications with a higher risk of premature death. At the same time, early diagnosis is based on relatively laboratory tests — serum calcium and parathyroid hormone, and visualization of the adenoma in most cases is provided by ultrasound examination and scintigraphy. This article is a case report of primary hyperparathyroidism caused by an ectopic parathyroid adenoma located in the mediastinum, when late treatment led to the patient’s long-term suffering and the development of severe complications.