Hypothalamus-Pituitary Axis

The hypothalamo-hypophyseal axis is the master regulatory hub of the endocrine system. The hypothalamus integrates neural and humoral signals and translates them into hormonal output via the anterior and posterior pituitary. This topic is exceptionally high-yield because the physiology directly underpins clinical entities asked across Medicine, Endocrinology, and Pharmacology — acromegaly, hyperprolactinaemia, SIADH, and diabetes insipidus.

Anatomical & developmental orientation

The pituitary (hypophysis) sits in the sella turcica of the sphenoid bone, connected to the hypothalamus by the infundibular stalk. It has two functionally and embryologically distinct lobes:

Feature	Anterior pituitary (adenohypophysis)	Posterior pituitary (neurohypophysis)
Embryological origin	Rathke's pouch (oral ectoderm)	Neural ectoderm (downgrowth of diencephalon)
Hormone synthesis	Synthesises its own hormones	Stores hormones made in hypothalamus
Connection to hypothalamus	Hypophyseal portal vessels (vascular)	Direct neuronal axons (hypothalamo-hypophyseal tract)
Hormones	GH, TSH, ACTH, FSH, LH, prolactin	ADH (vasopressin), oxytocin
Blood supply	Superior hypophyseal artery → portal system	Inferior hypophyseal artery

High-yield: The anterior pituitary has no direct arterial supply for hormone control — it receives blood that has first passed through the hypothalamic median eminence via the hypophyseal portal system. This is why hypothalamic releasing factors reach the adenohypophysis in high concentration.

The intermediate lobe (pars intermedia) is vestigial in adult humans but secretes MSH (melanocyte-stimulating hormone) derived from POMC.

The hypophyseal portal system

A portal system is two capillary beds in series connected by portal veins. In the median eminence, hypothalamic neurons release releasing/inhibiting hormones into the primary capillary plexus. These drain via long portal veins down the stalk to the secondary plexus in the adenohypophysis, where they act on specific cell types.

Hypothalamic neuron → median eminence primary plexus → long portal veins → anterior pituitary secondary plexus → endocrine cell

High-yield: Section of the pituitary stalk → fall in all anterior hormones EXCEPT prolactin, which RISES. Reason: prolactin is under tonic inhibition by dopamine; cutting the stalk removes dopamine, disinhibiting prolactin. This is the basis of the "stalk effect" in compressive sellar masses.

Hypothalamic releasing & inhibiting hormones

Hypothalamic hormone	Action on pituitary	Net effect
GHRH	↑ GH release	Growth, IGF-1
Somatostatin (GHIH)	↓ GH, ↓ TSH	Inhibitory
TRH	↑ TSH, ↑ prolactin	Thyroid + lactation
CRH	↑ ACTH (via POMC)	Cortisol axis
GnRH (pulsatile)	↑ FSH, ↑ LH	Reproduction
Dopamine (PIF)	↓ prolactin	Tonic inhibition

High-yield: GnRH must be PULSATILE to stimulate gonadotrophs. Continuous GnRH (or long-acting agonists like leuprolide) DOWN-regulates receptors and SUPPRESSES FSH/LH — exploited therapeutically in precocious puberty, prostate cancer, and endometriosis.

High-yield: TRH stimulates BOTH TSH and prolactin. In primary hypothyroidism, high TRH can cause hyperprolactinaemia and galactorrhoea — always check TSH before labelling a prolactinoma.

Mnemonic for anterior cell types & relative proportions — "GH PALS": Somatotrophs (GH, ~50%) are the most numerous, then corticotrophs, then lactotrophs, thyrotrophs and gonadotrophs.

Anterior pituitary hormones in detail

Growth hormone (GH)

• Secreted by somatotrophs; a single-chain peptide, structurally related to prolactin.
• Released in pulses, maximal during slow-wave (deep) sleep; stimulated by hypoglycaemia, exercise, stress, ghrelin, amino acids (arginine).
• Acts directly (anti-insulin, lipolytic, diabetogenic) and indirectly via hepatic IGF-1 (somatomedin C) for linear growth and protein anabolism.
• Negative feedback: IGF-1 and GH itself stimulate somatostatin and inhibit GHRH.

High-yield: GH is diabetogenic — it raises blood glucose. IGF-1 mediates growth; GH directly mediates the metabolic (anti-insulin) effects.

Prolactin

• From lactotrophs; the only anterior hormone under dominant inhibitory (dopaminergic) control.
• Stimulates milk synthesis; suckling is the main physiological stimulus (neuroendocrine reflex). High prolactin suppresses GnRH → lactational amenorrhoea.
• Oestrogen, TRH, stress, sleep, and pregnancy raise prolactin.

TSH, ACTH, FSH, LH

• TSH (thyrotrophs) → thyroid follicular cells; ACTH (corticotrophs, from POMC) → adrenal cortisol; FSH/LH (gonadotrophs) → gonadal steroidogenesis and gametogenesis.
• TSH, FSH, and LH (with hCG) are glycoprotein hormones sharing a common α-subunit; the β-subunit confers specificity.

High-yield: POMC is cleaved into ACTH, β-endorphin, β-lipotropin, and MSH. In Addison's/ectopic ACTH, excess POMC-derived MSH activity causes hyperpigmentation.

Posterior pituitary hormones

Both ADH and oxytocin are nonapeptides synthesised in supraoptic and paraventricular nuclei, transported down axons bound to neurophysins, and stored in the neurohypophysis.

	ADH (vasopressin)	Oxytocin
Main nucleus	Supraoptic > paraventricular	Paraventricular > supraoptic
Stimulus	↑ Plasma osmolality, ↓ blood volume/pressure	Suckling, cervical/uterine stretch (Ferguson reflex)
Receptors	V1 (vessels, Gq), V2 (kidney, Gs–cAMP)	Oxytocin receptor (Gq)
Renal action	Inserts aquaporin-2 in collecting duct → water reabsorption	—
Other actions	V1 vasoconstriction	Milk ejection (let-down), uterine contraction

High-yield: ADH acts on V2 receptors → cAMP → aquaporin-2 channel insertion into the apical membrane of the collecting duct principal cells. This is the molecular target deranged in nephrogenic DI.

Osmoreceptors in the anterior hypothalamus (organum vasculosum) sense plasma osmolality; the threshold for ADH release (~280 mOsm/kg) is lower than the thirst threshold (~290 mOsm/kg), so ADH defends against dehydration first.

Negative feedback control

Classic three-tier loops: Hypothalamus → Pituitary → Target gland → hormone → feedback inhibition of both higher centres.

• Long loop: target hormone (cortisol, thyroxine) inhibits hypothalamus + pituitary.
• Short loop: pituitary hormone inhibits its own hypothalamic releasing factor.
• Ultra-short loop: hypothalamic hormone inhibits its own secretion.

High-yield: This feedback logic underlies the distinction between primary (target gland failure → high pituitary hormone) and secondary/central (pituitary failure → low pituitary hormone) endocrine disease. E.g. primary hypothyroidism = low T4, high TSH; central hypothyroidism = low T4, low/inappropriately normal TSH.

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Clinical integrations

1. Acromegaly / Gigantism (GH excess)

Cause: usually a somatotroph adenoma of the anterior pituitary. Before epiphyseal fusion → gigantism; after fusion → acromegaly.

Clinical features: coarse facial features, frontal bossing, prognathism, macroglossia, soft-tissue overgrowth, large hands/feet (increasing ring/shoe size), carpal tunnel syndrome, skin tags, hyperhidrosis, obstructive sleep apnoea, impaired glucose tolerance/diabetes, hypertension, and cardiomyopathy (leading cause of death).

Diagnosis flow:

1. Screen with serum IGF-1 (best single screening test — stable, no diurnal variation).
2. Confirm with Oral Glucose Tolerance Test (OGTT): failure of GH to suppress below 1 ng/mL (or <0.4 ng/mL by ultrasensitive assay) after 75 g glucose is diagnostic — because in normal subjects glucose suppresses GH.
3. MRI pituitary to localise the adenoma.

High-yield: Investigation of choice for acromegaly = OGTT with GH suppression (random GH is unreliable due to pulsatility). IGF-1 = best screening test.

Management:

• Trans-sphenoidal surgery = treatment of choice for most.
• Medical: somatostatin analogues (octreotide, lanreotide) = first-line drug therapy; cabergoline (dopamine agonist) adjunct; pegvisomant (GH-receptor antagonist) for resistant cases.
• Radiotherapy if refractory.

2. Hyperprolactinaemia

Causes: prolactinoma (commonest functional pituitary tumour), stalk-effect from any sellar mass, drugs (dopamine antagonists — antipsychotics, metoclopramide, domperidone), primary hypothyroidism, pregnancy, chronic kidney disease, oestrogens.

Features: women — galactorrhoea, oligo/amenorrhoea, infertility; men — decreased libido, erectile dysfunction, gynaecomastia. Macroadenomas → bitemporal hemianopia (optic chiasm compression) and headache.

High-yield: A microprolactinoma typically gives prolactin >200 ng/mL, whereas mild elevations (25–100 ng/mL) suggest stalk effect, drugs, or hypothyroidism. Always exclude pregnancy, hypothyroidism, and drugs first.

Management: Dopamine agonist = drug of choice EVEN for macroadenomas — cabergoline (preferred, fewer side-effects) or bromocriptine. Surgery reserved for drug resistance/intolerance.

High-yield: Unlike most pituitary tumours, prolactinomas are managed medically first because they shrink dramatically with dopamine agonists.

3. SIADH (Syndrome of Inappropriate ADH secretion)

Pathophysiology: excess ADH → water retention → euvolaemic (dilutional) hyponatraemia with inappropriately concentrated urine.

Causes (mnemonic — think SIADH): Small-cell lung carcinoma (ectopic ADH), CNS insults (meningitis, stroke, trauma), pulmonary disease (pneumonia, TB), drugs (carbamazepine, SSRIs, cyclophosphamide, vincristine), post-operative pain/nausea.

Diagnostic criteria (Bartter–Schwartz):

Parameter	Finding in SIADH
Serum sodium	< 135 mmol/L (hypotonic, serum osm < 275)
Urine osmolality	> 100 mOsm/kg (inappropriately concentrated)
Urine sodium	> 40 mmol/L (with normal salt intake)
Volume status	Euvolaemic (no oedema, no hypotension)
Thyroid/adrenal/renal function	Normal
Uric acid	Typically low

Management flow:

1. Fluid (water) restriction = first-line.
2. Hypertonic (3%) saline ONLY for severe symptomatic hyponatraemia (seizures, coma).
3. Vaptans (tolvaptan — V2 antagonist) or demeclocycline for chronic refractory cases.

High-yield: Correct sodium SLOWLY — < 8–10 mmol/L per 24 h. Rapid correction risks osmotic demyelination syndrome (central pontine myelinolysis). Conversely, overly rapid correction of hyponatraemia is the danger; rapid correction of hypernatraemia causes cerebral oedema.

4. Diabetes Insipidus (DI)

Polyuria + polydipsia with dilute urine due to ADH deficiency (central) or renal resistance (nephrogenic).

Feature	Central (cranial) DI	Nephrogenic DI
Defect	↓ ADH secretion	ADH present, renal V2 resistance
Causes	Tumour, trauma, surgery, idiopathic	Lithium, hypercalcaemia, hypokalaemia, hereditary
Plasma ADH	Low	High/normal
Desmopressin response	Urine concentrates (>50% rise in osm)	No response
Treatment	Desmopressin (DDAVP)	Treat cause; thiazides + low salt, amiloride (Li-induced), NSAIDs

Water deprivation test flow: Deprive water → measure urine osmolality → if stays dilute, give desmopressin → urine concentrates = central DI; no change = nephrogenic DI.

High-yield: In primary polydipsia (psychogenic), water deprivation does concentrate urine (kidney works, ADH intact) — distinguishing it from true DI. Copeptin (a stable surrogate for ADH) assay is now increasingly used.

Pituitary apoplexy & Sheehan syndrome

• Pituitary apoplexy: sudden haemorrhage/infarction of a pituitary tumour → thunderclap headache, ophthalmoplegia, visual loss, acute hypopituitarism — an endocrine emergency requiring hydrocortisone.
• Sheehan syndrome: postpartum pituitary necrosis from severe haemorrhage/hypotension; failure of lactation is the earliest sign (prolactin deficiency).

Key differentials & integration pitfalls

• Galactorrhoea: prolactinoma vs primary hypothyroidism vs drugs — check TSH and prolactin together.
• Hyponatraemia: SIADH (euvolaemic) vs cerebral salt wasting (hypovolaemic, high urine Na) vs hypothyroidism/Addison's. Volume status is the key discriminator.
• Polyuria: DI vs diabetes mellitus (osmotic diuresis, high glucose) vs primary polydipsia.
• Visual field defect: bitemporal hemianopia localises to chiasmal compression by a pituitary macroadenoma.

Recently asked / exam angle

• OGTT-GH suppression as the confirmatory test for acromegaly, and IGF-1 as screening — repeatedly tested distinction.
• Stalk section → prolactin rises, all others fall — classic single-best-answer.
• GnRH must be pulsatile; continuous agonist suppresses gonadotrophins (leuprolide mechanism).
• ADH → V2 → aquaporin-2 molecular pathway; lithium causing nephrogenic DI.
• Bartter–Schwartz criteria for SIADH, and low serum uric acid as a clue.
• Cabergoline first-line for prolactinoma even when it is a macroadenoma.
• TRH stimulates prolactin → hyperprolactinaemia in hypothyroidism.
• Common α-subunit shared by TSH, FSH, LH, hCG.
• Osmotic demyelination from rapid hyponatraemia correction (assertion–reason format).
• Sheehan syndrome presenting with failure of lactation postpartum.

Rapid revision

1. Anterior pituitary = Rathke's pouch; posterior = neural ectoderm.
2. Stalk cut → prolactin ↑, everything else ↓ (loss of dopamine inhibition).
3. GnRH pulsatile stimulates; continuous suppresses FSH/LH.
4. IGF-1 = screen, OGTT-GH suppression = confirm acromegaly.
5. Somatostatin analogue (octreotide) = first-line drug; surgery = definitive in acromegaly.
6. Cabergoline = drug of choice for prolactinoma, even macroadenomas.
7. TRH ↑ prolactin → primary hypothyroidism can mimic prolactinoma.
8. ADH → V2 receptor → cAMP → aquaporin-2 in collecting duct.
9. SIADH: euvolaemic hyponatraemia, urine osm >100, urine Na >40, low uric acid; treat with fluid restriction.
10. Lithium = classic nephrogenic DI; treat with thiazides/amiloride. Central DI → desmopressin.
11. Correct hyponatraemia slowly (<8–10 mmol/L/day) to avoid central pontine myelinolysis.
12. Sheehan syndrome → postpartum failure of lactation; pituitary apoplexy → give hydrocortisone urgently. Bitemporal hemianopia = chiasmal compression.