Water balance in the newborn is controlled primarily by which hormone?

• A. Antidiuretic hormone (ADH)
• B. Aldosterone
• C. Sodium
• D. Renin

Answer: (A)

Water balance in the newborn is controlled primarily by antidiuretic hormone (ADH). ADH acts on the kidney’s collecting ducts to reabsorb free water, helping to maintain serum osmolality and intravascular volume. It’s released in response to higher plasma osmolality or lower blood volume, and it increases water permeability by promoting aquaporin-2 channels in the collecting ducts. Aldosterone mainly affects sodium reabsorption and thus extracellular fluid volume but does not directly regulate water balance. The renin-angiotensin system influences blood pressure and sodium handling, not the primary mechanism of water conservation. In newborns, while renal concentrating ability is still maturing, ADH remains the primary hormone governing water homeostasis.