Diabetes insipidus is caused by which of the following?

• A. Insulin deficiency
• B. Adrenal insufficiency
• C. Thyroid hormone deficiency
• D. Antidiuretic hormone is not secreted adequately, or the kidney is resistant to its effect

Answer: (D)

Diabetes insipidus stems from a problem with antidiuretic hormone, also called vasopressin. Normally, ADH is released by the posterior pituitary in response to higher plasma osmolality and acts on the kidney’s collecting ducts to insert water channels, allowing water to be reabsorbed. If ADH is not secreted enough (central DI) or the kidneys don’t respond to it (nephrogenic DI), the kidneys can’t reabsorb as much water. That leads to large volumes of dilute urine and increased thirst. This distinction is why this condition is not caused by insulin deficiency, adrenal insufficiency, or thyroid hormone deficiency—their effects don’t produce the same failure to concentrate urine due to ADH signaling.