MRCOG Part 1 · Endocrinology · Hypothalamus and Pituitary Gland

Posterior Pituitary Hormones: MRCOG Part 1 questions and key facts

The posterior pituitary hormones, arginine vasopressin and oxytocin, are peptide hormones synthesised in the hypothalamic supraoptic and paraventricular nuclei and released into the circulation from the posterior pituitary (neurohypophysis), which acts as a storage site. Arginine vasopressin regulates intravascular volume and osmolality by inserting aquaporin-2 channels into renal collecting ducts to promote free water reabsorption, while inappropriate (often ectopic) secretion causes the syndrome of inappropriate antidiuretic hormone secretion (SIADH) with dilutional hyponatraemia. Oxytocin acts directly on smooth muscle and contractile tissues. It stimulates mammary myoepithelial cell contraction to drive the milk ejection reflex during lactation, and mediates powerful postpartum myometrial contractions that promote uterine involution and constrict the placental bed to prevent haemorrhage.

The Uterio bank has 13 questions on this topic. 2 arebelow in full; the syllabus detail is in the Endocrinology study guide.

How the exam tests Posterior Pituitary Hormones

  • Distinguishing oxytocin's role in milk ejection via myoepithelial contraction from prolactin's role in milk synthesis.
  • Arginine vasopressin renal action through aquaporin-2 insertion in collecting ducts, driving free water reabsorption.
  • Uterine myometrial contraction driven by oxytocin to facilitate postpartum involution and control placental site bleeding.
  • Paraneoplastic syndrome of inappropriate antidiuretic hormone secretion, characteristically associated with small cell lung carcinoma.

High-yield facts

  1. Arginine vasopressin and oxytocin are synthesised in the hypothalamic supraoptic and paraventricular nuclei and stored in and released from the posterior pituitary.
  2. Arginine vasopressin increases water reabsorption in renal collecting ducts by inserting aquaporin-2 channels, leading to concentrated urine.
  3. Arginine vasopressin also has vasoconstrictor properties (V1 receptors); its renal action is on water, and aldosterone, not AVP, is the principal regulator of sodium and potassium handling.
  4. The syndrome of inappropriate antidiuretic hormone secretion causes excessive water retention and dilutional hyponatraemia.
  5. Small cell lung cancer is the most frequent malignant cause of SIADH, as a paraneoplastic syndrome from ectopic ADH secretion by neuroendocrine tumour cells.
  6. Central nervous system infections, space-occupying lesions, surgery, and Guillain-Barré syndrome can trigger the syndrome of inappropriate antidiuretic hormone secretion.
  7. Medications including sulfonylureas, selective serotonin reuptake inhibitors, and tricyclic antidepressants are recognised causes of inappropriate antidiuretic hormone secretion.
  8. Oxytocin is released from the posterior pituitary gland and binds to myometrial receptors to stimulate coordinated uterine contractions.
  9. Postpartum oxytocin-induced contractions promote uterine involution and constrict placental site blood vessels to minimise haemorrhage.
  10. Oxytocin release is triggered by suckling, stimulating the contraction of mammary myoepithelial cells surrounding alveoli to drive the milk ejection reflex.
  11. Prolactin stimulates breast milk production, whereas oxytocin facilitates milk ejection into the mammary ducts.

Common pitfalls

  • Confusing oxytocin's role in milk ejection via myoepithelial contraction with prolactin's primary role in milk synthesis.
  • Choosing a sodium or potassium effect as the primary renal action of arginine vasopressin; its primary action is water reabsorption, whereas aldosterone is the principal regulator of sodium and potassium.
  • Assuming arginine vasopressin causes renal vasodilation, whereas it actually exhibits vasoconstrictor properties.

Practice questions: Posterior Pituitary Hormones

Single best answer, exactly as the exam asks. Commit to an option before opening the explanation.

Question 1

Endocrinology

A patient presents with hyponatraemia and is subsequently diagnosed with SIADH. Which of the following malignancies is MOST frequently implicated as the underlying cause of this paraneoplastic presentation?

  1. AAppendiceal carcinoid tumour
  2. BSmall cell lung cancer
  3. CFibrosarcoma of the uterus
  4. DOvarian fibroma
  5. EPrimary liver cancer
Show answer and explanation

Correct answer: B. Small cell lung cancer

Key takeaway

SIADH is when there is too much ADH (vasopressin), which causes the body to retain water, diluting the blood and causing hyponatraemia. The most common underlying cause in malignancy-associated SIADH is small cell lung cancer.

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) causes euvolaemic hyponatraemia through water retention. Its causes include central nervous system disorders (for example infection, space-occupying lesions and Guillain–Barré syndrome), drugs (for example sulfonylureas, SSRIs and tricyclic antidepressants), the postoperative state and malignancy, in which it arises as a paraneoplastic syndrome: an effect of the tumour remote from its site.

Small cell lung cancer is the most frequent cause of paraneoplastic SIADH, accounting for most malignancy-associated cases, and SIADH develops in a substantial minority of patients with this tumour. Small cell carcinomas are neuroendocrine tumours and frequently secrete peptide hormones, including antidiuretic hormone (ADH, vasopressin) and ACTH.

The other tumours listed have a much weaker association. An Appendiceal carcinoid tumour is a neuroendocrine tumour that may secrete several hormones, but it is a rare cause of SIADH. Fibrosarcoma of the uterus and Ovarian fibroma can occasionally produce paraneoplastic effects, but SIADH is very rare with either. Primary liver cancer is associated with paraneoplastic hypoglycaemia and hypercalcaemia but is less commonly linked to SIADH.

Question 2

Endocrinology

What is the primary action of arginine vasopressin (AVP) on the kidneys?

  1. ADecreased sodium reabsorption
  2. BIncreased potassium secretion
  3. CIncreased sodium secretion
  4. DIncreased water reabsorption
  5. EVasodilation of renal arterioles
Show answer and explanation

Correct answer: D. Increased water reabsorption

Key takeaway

Arginine vasopressin (AVP), or antidiuretic hormone (ADH), primarily acts on the kidneys to increase water reabsorption, leading to concentrated urine and water conservation.

Arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), is primarily responsible for regulating the body's water balance.

The principal renal action of AVP is increased water reabsorption. AVP acts on the collecting ducts of the nephrons, increasing their permeability to water by inserting aquaporin-2 channels. This allows water to be reabsorbed from the tubular fluid back into the bloodstream, concentrating the urine and conserving water.

AVP is not the main regulator of sodium or potassium handling; aldosterone is the primary hormone regulating sodium reabsorption and potassium secretion, so increased potassium secretion is not the primary action of AVP, whose effect on potassium is minor. Where AVP does affect sodium transport, by stimulating the epithelial sodium channel (ENaC) in the collecting duct through its V2 receptors, it modestly increases sodium reabsorption and reduces sodium excretion, the opposite of decreased sodium reabsorption or increased sodium secretion. Vasodilation of renal arterioles is also the opposite of what happens: AVP has vasoconstrictor properties.

11 more posterior pituitary hormones questions in the bank

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What the syllabus covers under posterior pituitary hormones

  • Arginine Vasopressin (AVP)
  • Oxytocin (OXY)

Read the full section in the Endocrinology guide, or check the next exam dates.