Severe Hyperparathyroidism in a 3-year-old girl Due to Homozygous Inactivating Variant of the <i>Calcium Sensing Receptor</i> Gene

Homozygous inactivating pathogenic variants of calcium-sensing receptors cause severe hyperparathyroidism (HPT), typically presenting in early infancy. In the pediatric age group, HPT is uncommon and can be due to parathyroid adenoma or hyperplasia. We describe the case of a 3-year, 10-month-old girl who presented with severe HPT with symptomatic hypercalcemia, skeletal demineralization, and bone pains resulting from a homozygous missense variant <i>p.Gly670Arg</i> in the <i>CaSR</i> gene that was previously reported in association with familial hypocalciuric hypercalcemia in its heterozygous state. Parathyroidectomy with autotransplant of one-fourth of the gland in the forearm did not result in a cure even after the removal of autotransplant. In addition to controlling preoperative hypercalcemia, pamidronate helped alleviate bone pains and improve skeletal remineralization when used postoperatively for a brief period.