As hypocalcemia is uncommon, serum calcium levels are not routinely measured in many emergency medicine clinics. We report a case of an adolescent girl with a transient loss of consciousness due to hypocalcemia. A 13-year-old healthy girl had a syncopal episode complicated with numbness in the extremities. On admission, she was fully conscious, but hypocalcemia and QT prolongation were noted. After careful consideration of the possible etiologies, the patient was diagnosed with acquired QT prolongation due to primary hypoparathyroidism. The patient's serum calcium levels were controlled by activated vitamin D and calcium supplementation. Primary hypoparathyroidism-associated hypocalcemia can cause QT prolongation and neurological complications, even in previously healthy adolescents.