Neonatal Severe Primary Hyperparathyroidism (NSHPT) is a rare, life-threatening disorder characterized by very high serum calcium concentrations, skeletal demineralization, and multi-glandular parathyroid hyperplasia. Infants present with lethargy, hypotonia, failure to thrive, bony undermineralization, multiple fractures, and severe skeletal deformities, including thoracic narrowing that can lead to respiratory disease. Symptoms occur before the age of 6 months and, if left untreated, can lead to florid rickets, devastating neurodevelopmental disorders, and often fatality. Usually a parathyroidectomy is necessary within the first few weeks of life to prevent death. Of note, there have been milder, transient cases reported with a lack of family history of FHH or secondary hyperparathyroidism. NSHPT is inherited in an autosomal recessive manner, and affected infants are usually born to parents with FHH who may be consanguineous. Milder, transient neonatal disease may be seen when one or both of the parents are symptomatic. In rare cases, a single heterozygous mutation has been reported to act in a dominant-negative fashion. Paternal and de novo mutations tend to have a less severe clinical course due to the influence of maternal-fetal calcium regulations.