Case Study: Overlooking minor issues can lead to significant losses, even in Korea, in which medical standards are high and access to medical care is easy. Here, we present a case in which calcium level tests were not performed continuously, leading to Fahr’s disease due to secondary hypoparathyroidism. Although Fahr’s disease secondary to hypoparathyroidism is rare, it has been documented in the literature. However, reports that incorporate a differential diagnosis via genetic testing or exclusion of Parkinson’s disease remain limited. This report details our diagnostic and therapeutic approach to contribute to clinical knowledge and support other clinicians in the recognition and management of similar cases. A 48-year-old woman diagnosed with Parkinson’s disease six months previously was admitted to a local medical center with recurrent seizures. Computed tomography demonstrated extensive brain calcifications involving the basal ganglia, cerebral cortex, dentate nucleus, and subcortical white matter. Therefore, the patient was diagnosed with Fahr’s disease. The patient’s condition worsened, prompting transfer to the hospital. As the patient emerged from a semi-comatose state, she exhibited persistently slurred speech, disorientation, and dystonia. Severe hypocalcemia was observed. Despite a history of thyroidectomy for thyroid cancer 20 years previously, no related tests had been performed. Next-generation sequencing targeting genes associated with Fahr’s disease (PDGFB, PDGFRB, SLC20A2, and XPR1) revealed no pathogenic variants. Additionally, brain positron emission tomography (PET) revealed no evidence of Parkinson’s disease. After calcium and vitamin D supplementation, she was discharged in a stable condition. Clinically, the patient demonstrated significant improvement with the resolution of mental confusion, dystonia, and gait disturbances. She is currently being followed in the outpatient clinic with no signs of worsening neurological symptoms after one year. Timely correction of calcium levels in patients with hypocalcemia is crucial. Furthermore, in patients presenting with neurological symptoms, clinicians should consider primary neurological disorders and secondary causes.