PhysiologyTier 2Medical Sciences concept

Posterior pituitary hormones

Core concept

The posterior pituitary does not synthesise hormones - it stores and releases hormones made in the hypothalamus (supraoptic and paraventricular nuclei), delivered via the hypothalamo-hypophyseal tract
Two hormones: ADH (vasopressin) and oxytocin
ADH acts on V2 receptors (renal collecting duct) -> aquaporin-2 insertion -> water reabsorption; V1 receptors -> vasoconstriction
Oxytocin drives uterine contraction (labour) and the milk ejection (let-down) reflex

ADH release stimulated by: rising plasma osmolality (most sensitive trigger, via hypothalamic osmoreceptors), falling blood volume/pressure (baroreceptors - less sensitive but overrides osmotic control in severe hypovolaemia), nausea, pain, nicotine
Oxytocin release is a positive feedback loop - suckling/cervical stretch -> more oxytocin -> stronger contraction/ejection
SIADH = excess ADH action -> euvolaemic hyponatraemia; diabetes insipidus = deficient ADH action (central = deficient secretion; nephrogenic = renal resistance)

Desmopressin (DDAVP) - synthetic, V2-selective ADH analogue - used in central DI, and (paradoxically) in mild von Willebrand disease/haemophilia A, where it releases stored vWF/factor VIII from endothelium
Synthetic oxytocin (Syntocinon) - induction/augmentation of labour, prevention of postpartum haemorrhage
Pituitary stalk/posterior lobe lesions cause DI as a distinct pattern from anterior pituitary hormone loss

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