Rare cause of life-threatening hypercalcemia in an infant: a case report

National audience; BACKGROUND: 25-Hydroxyvitamin D 24-hydroxylase (CYP24A1) deficiency is a rare cause of autosomal recessive infantile hypercalcemia due to vitamine D hypersensitivity. CASE PRESENTATION: We report the case of a 2-year-old boy who presented with severe hypercalcemia-hypercalciuria and a bilateral nephrocalcinosis. Laboratory investigations detected a collapsed parathormone and a highly elevated 1α,25-dihydroxycholecalciferol along with an increased phosphate excretion (hypophosphatemia and hyperphosphaturia). An adapted management with two courses of palmidronic acid and an eviction of vitamin D and calcium allowed to stabilize him. A homozygous p.Leu409Ser pathogenic variant on CYP24A1 gene resulting in a collapsed 25-Hydroxyvitamin D24-hydroxylase activity was found. A normal development is possible with a meticulous clinical, biological and nutritional management and monitoring. CONCLUSIONS: Vitamin D hypersensitivity is challenging during childhood, especially due to the need to avoid vitamin D while requiring a close nutritional monitoring to maintain a normal growth. Biomarkers such as vitamin D metabolite ratio and 24,25(OH)2D3 along with ionized calcium and nutritional management can contribute to properly follow patients with vitamin D hypersensitivity.