Safety and Efficacy of Chlorthalidone in Type 1 Diabetes

Inclusion Criteria: * Diagnosis of T1D * Age 12-21 years * Tanner Stage 2 or greater pubertal development * Urine calcium excretion ≥ 4 mg/kg/day * Able to swallow pills Exclusion Criteria: * BMI \> 99th percentile for age (\<18 years) or BMI \>35 kg/m2 (≥ 18 years) * Coexistent conditions that may affect calcium metabolism including: * celiac disease * Graves' Disease * Addison's disease * hypo- or hyperparathyroidism * History of diabetes related complications including: * neuropathy * retinopathy * nephropathy * gastroparesis * History of oral or inhaled corticosteroid use for ≥ 5 consecutive days within the past month * History of any diuretic use within the past month * Laboratory abnormalities on screening bloodwork including: * estimated glomerular filtration rate \<90 mL/min per 1.73 m2 of body surface area * serum calcium \>10.5 mg/dL * serum potassium \<3.5 mmol/L * Systolic or diastolic blood pressure \<5th percentile for age and sex50 for age \<18 years or systolic \<90 mmHg or diastolic blood pressure \<60 mmHG for age ≥18 years