Lack of effect in initial subjects

The purpose of this study is to determine whether metformin can increase urine concentration (osmolality) and decrease the amount of urine in patients with congenital nephrogenic diabetes insipidus (NDI).

Nephrogenic diabetes insipidus (NDI) is a genetic disease. Patients with this disease make large amounts of urine because their kidneys don't hold on to water. The large amount of urine means that patients need to urinate very frequently. They are also at increased risk for dehydration if they don't drink enough. The large amount of urine can sometimes damage their bladders and kidneys. There are some medicines that may help these patients urinate less, but they are not very effective. There is evidence in animal studies that a medication called metformin may help patients with NDI urinate less.

Inclusion Criteria:

• Males with a documented mutation in the vasopressin type 2 receptor (V2R)
• Willing to provide consent and/or assent as appropriate
• Capable of providing urine samples as dictated by the protocol

Exclusion Criteria:

• Urinary incontinence
• Subjects who have heart disease, liver disease, diabetes, cancer, or other significant disease other than Nephrogenic Diabetes Insipidus (NDI)
• Subjects with significant renal dysfunction (defined as a calculated glomerular filtration rate (GFR) <80 ml/min/1.73 m^2)
• Subjects with acquired NDI