What hormone is deficient in central diabetes insipidus and what drug is used diagnostically to test for DI?

• A. GH deficiency; somatostatin.
• B. Insulin deficiency; glucose.
• C. ADH (vasopressin) deficiency; desmopressin (DDAVP) is used diagnostically.
• D. ACTH deficiency; cosyntropin.

Answer: (C)

Central diabetes insipidus happens when there isn’t enough antidiuretic hormone (ADH, also called vasopressin) from the posterior pituitary. ADH’s job is to act on the kidney’s collecting ducts and promote water reabsorption by opening aquaporin channels; without it, urine becomes very dilute and produced in large volumes, while thirst drives up serum osmolality.

To figure out if DI is due to ADH deficiency (central) or kidney insensitivity (nephrogenic), clinicians use a desmopressin (DDAVP) challenge. Desmopressin is a synthetic vasopressin analog that mimics ADH. In central DI, giving DDAVP markedly concentrates the urine and reduces urine volume because the kidneys now respond to the missing hormone. In nephrogenic DI, the response is minimal because the kidneys don’t respond to ADH anyway. So the hormone deficient is ADH, and the diagnostically used drug is desmopressin (DDAVP).