A novel homozygous inactivating mutation of the calcium-sensing receptor causing familial hypocalciuric hypercalcemia complicated with primary hyperparathyroidism due to parathyroid adenoma: A case report.

Familial hypocalciuric hypercalcemia is an autosomal dominant genetic disorder characterized by mild to moderate hypercalcemia, mild hypermagnesemia, and normal or inappropriately elevated parathyroid hormone levels. Familial hypocalciuric hypercalcemia 1 is the most prevalent form of familial hypocalciuric hypercalcemia and is typically caused by heterozygous loss-of-function mutations in the calcium-sensing receptor (CaSR) gene. Homozygous CaSR mutations are more commonly associated with neonatal severe primary hyperparathyroidism. We report the case of a female patient in her early 40s harboring a novel homozygous frameshift mutation in the CaSR gene (c.2603_2604insTT), resulting in familial hypocalciuric hypercalcemia 1. The patient presented with persistent hypocalciuria, hypercalcemia, and primary hyperparathyroidism, along with a family history of consanguinity. This case highlights the phenotypic variability associated with CaSR mutations and broadens the clinical spectrum of homozygous CaSR-related disorders. Increased clinical awareness of atypical genetic presentations is essential to avoid misdiagnosis and to ensure appropriate management of patients with familial disorders of calcium homeostasis.