Idiopathic infantile hypercalcemia is caused by loss-of-function mutations in the CYP24A1 gene, which encodes an enzyme that degrades vitamin D. The disease is characterized by nonspecific symptoms. In this case study, the infant merely presents with failure to thrive and dehydration, giving rise to a broad differential diagnosis. Ultrasound additionally revealed bilateral nephrocalcinosis. Genetic research found two compound mutations in the CYP24A1 gene. Following a diet low in calcium and vitamin D and starting calcium-lowering therapy led to adequate weight gain and normalized calcium levels. The case highlights the importance of determining full serum electrolytes in children with faltering growth.