Gestational DI is caused by which mechanism?

• A. Vasopressinase
• B. Desmopressin deficiency
• C. AVP excess
• D. Secondary to dehydration

Answer: (A)

Gestational diabetes insipidus arises because the placenta makes a vasopressin-degrading enzyme, vasopressinase. This increases breakdown of antidiuretic hormone (AVP) during pregnancy, so there isn’t enough AVP to signal the kidneys to conserve water. The result is large volumes of dilute urine and thirst. After delivery, the placental source of vasopressinase disappears, and AVP action returns to normal, resolving the DI.

Desmopressin deficiency would cause DI but isn’t specific to pregnancy and isn’t the mechanism here; AVP excess would reduce urine output rather than cause DI; dehydration can cause polyuria but doesn’t explain the pregnancy-specific loss of AVP activity.