Calcitriol Monotherapy for X-Linked Hypophosphatemia

Inclusion Criteria: * Clinical diagnosis of XLH with family history excluding male-to-male transmission, or positive genotype for PHEX mutation * Serum PTH levels less than 1.5x the upper limit of normal * Serum calcium levels less than 10.0 mg/dl * eGFR \>= 60 mL/min/1.73m2 * 25(OH) vitamin D level \>= 20 ng/dL Exclusion Criteria: * Known allergy to calcitriol * Pregnancy or breast feeding * Use of skeletally active agents such as bisphosphonates, teriparatide, SERMS, hormone replacement therapy and progesterone-only contraceptive agents (combination oral contraceptive use in premenopausal women is not an exclusion criterion). * Unwilling or unable to stop therapy with calcitriol and phosphate therapy for two weeks prior to study * Therapy with cinacalcet within the past two weeks * Current use of growth hormone therapy * Use of diuretics or medications that alter renal handling of mineral ions. * Use of glucocorticoids for more than 14 days in the past 12 months with the exception of inhaled agents. * History of malignancy except basal and squamous cell carcinoma of the skin. * Significant history of psychiatric disease per DSM-5. * Substance use disorder per DSM-5. * Significant cardiopulmonary disease (unstable CAD or stage D ACC/AHA heart failure). * Absence of laboratory values for serum calcium, phosphate and creatinine in the 24 months prior to enrollment.