Diabetes Insipidus II: Pathophysiology

ADH, synthesized in the hypothalamus and released from the posterior pituitary, acts on the distal nephron to promote water reabsorption. ADH inserts aquaporin-2 channels into collecting duct cells, allowing water to pass through and urine to become concentrated. This mechanism maintains plasma osmolality within the normal range of 285 to 295 milliosmoles per kilogram.

In central diabetes insipidus, damage to the hypothalamus or posterior pituitary reduces or abolishes ADH secretion. In nephrogenic diabetes insipidus, ADH is produced normally, but the kidneys fail to respond due to receptor or post-receptor defects. Both forms result in collecting ducts remaining impermeable to water, causing excessive urine loss.

Without effective ADH action, collecting ducts cannot reabsorb water, leading to polyuria—typically more than three liters per day of dilute urine. This excessive water loss raises plasma osmolality above normal, triggering the hypothalamic thirst center and causing intense polydipsia. If fluid intake cannot match urinary losses, dehydration develops rapidly.

Diabetes insipidus presents with low urine osmolality below 200 milliosmoles per kilogram, low urine specific gravity around 1.003, and elevated serum sodium above 145 milliequivalents per liter. Plasma osmolality rises above the normal range of 285 to 295 milliosmoles per kilogram. These findings reflect the kidneys' inability to concentrate urine and the resulting hypernatremia.

Osmoregulation occurs when hypothalamic osmoreceptors detect elevated plasma osmolality and trigger ADH release to conserve water. Baroregulation activates when vascular stretch receptors sense decreased blood volume or pressure, also stimulating ADH secretion. In diabetes insipidus, these feedback systems function normally but remain ineffective due to absent ADH or renal unresponsiveness to ADH.

Severe diabetes insipidus causes dehydration symptoms including dry mucous membranes, poor skin turgor, irritability, and confusion. Severe hypernatremia may trigger neurological complications such as seizures, coma, and circulatory collapse. In children, nocturnal enuresis or bedwetting may be an early presenting sign of the condition.

Gestational diabetes insipidus occurs when excess placental vasopressinase accelerates ADH breakdown during pregnancy, reducing hormone availability. Like other forms, it results in collecting ducts remaining impermeable to water and causes polyuria and polydipsia. This type is typically reversible after delivery when placental vasopressinase production ceases.