Diabetes insipidus results from deficiency of which posterior pituitary hormone?

• A. oxytocin
• B. ACTH
• C. ADH
• D. TSH

Answer: (C)

Diabetes insipidus stems from a deficiency of antidiuretic hormone, the posterior pituitary hormone also known as vasopressin. ADH normally acts on the kidneys to promote water reabsorption in the collecting ducts, making urine more concentrated and reducing volume. When ADH is deficient, the kidneys can’t concentrate urine, leading to large volumes of dilute urine and increased thirst. The other hormones listed are not the posterior pituitary hormone responsible for water balance: oxytocin is involved in uterine contractions and milk letdown, while ACTH and TSH come from the anterior pituitary and regulate cortisol and thyroid hormone production, respectively. Thus, the missing hormone in diabetes insipidus is ADH.