This is the case of a 76-year-old man admitted to hospital in a delirium state, previously diagnosed with a major depressive disorder at an age of 50 years, treated for years for chronic tension headache. The computed tomography of the head resulted negative. Inpatient laboratory tests revealed a mild hypercalcemia. Due to the progression of the disease (delirium state, dementia, tension headache, and depression), he was again admitted to hospital. The patient showed dysarthria, postural tremors, mirror movements and palmar hyperhidrosis, mild ataxia when walking, and rigidity. Sleep disturbances were also observed. He underwent several clinical diagnostic tests, which resulted negative. After more than 2 years, the ultrasound of the neck identified enlarged parathyroid glands. The patient was surgically treated, and three parathyroid glands were removed. Parathyroidectomy and lithium treatment resulted in improvement of cognitive functions. In elderly patients, concomitant presence of cognitive dysfunction may mask the underlying primary hyperparathyroidism.