Neuroendocrinology
Neuroendocrinology
What You'll Learn
- Hypothalamic-pituitary axis — hypothalamus releases hormones into the portal system → anterior pituitary; posterior pituitary stores ADH (supraoptic nucleus) and oxytocin (paraventricular nucleus)
- Hypothalamic hormones — TRH, CRH, GnRH, GHRH, somatostatin, dopamine (prolactin-inhibiting factor); dopamine and somatostatin are the two major inhibitory hypothalamic hormones (dopamine inhibits prolactin; somatostatin inhibits GH and TSH)
- Diabetes insipidus — central (low ADH) vs nephrogenic (ADH resistance); differentiate by desmopressin response
- SIADH — euvolemic hyponatremia with low serum osmolality, high urine osmolality, high urine Na+; common causes include CNS lesions, small cell lung cancer, and drugs (carbamazepine, SSRIs)
- Pituitary adenomas — prolactinoma is most common; macroadenomas compress optic chiasm → bitemporal hemianopia; treat prolactinomas medically (cabergoline), others surgically
- Neurological endocrine disorders — Hashimoto encephalopathy, myxedema coma, thyrotoxic periodic paralysis, Addisonian crisis, pheochromocytoma
- Paraneoplastic endocrine syndromes — ectopic ACTH and SIADH (small cell lung cancer), anti-NMDAR encephalitis (ovarian teratoma), limbic encephalitis
HighYield Pearls
- Stalk effect vs prolactinoma: stalk effect usually produces PRL <100–150 ng/mL; PRL >200 ng/mL strongly suggests prolactinoma; 100–200 is a gray zone. In large macroadenomas with modest PRL, repeat PRL with dilution to exclude hook effect before calling the mass nonfunctioning. Treatment differs (surgery for true non-functioning mass vs cabergoline for prolactinoma).
- SIADH vs CSW: both hyponatremic with high urine Na, but SIADH is euvolemic and treated with fluid restriction; CSW is volume-depleted (post-SAH) and worsens with restriction — replace Na AND volume with hypertonic + isotonic ± fludrocortisone.
- Correct hyponatremia slowly: ≤ 8–10 mEq/L per 24 h to avoid osmotic demyelination (central pontine myelinolysis) — especially in chronic SIADH, alcoholics, malnourished.
- Pituitary apoplexy = neurologic emergency: sudden severe headache + ophthalmoplegia (CN III/IV/VI in cavernous sinus) + bitemporal visual loss ± altered mental status. Stabilize + give stress-dose hydrocortisone promptly when adrenal insufficiency is possible. Urgent transsphenoidal decompression for worsening visual deficits, reduced consciousness, or severe neuro-ophthalmic compromise; stable patients may be observed with close endocrine/neurosurgical follow-up.
- Central DI after TBI/pituitary surgery: often a clinical inpatient diagnosis — hypotonic polyuria with rising serum sodium / osmolality; treat central with desmopressin; nephrogenic (lithium, hypercalcemia, hypokalemia) does NOT respond to DDAVP. Water deprivation or copeptin-based testing is for stable diagnostic uncertainty, NOT unstable acute hypernatremic polyuria (water deprivation can be unsafe in that setting).
- Acute adrenal insufficiency: hypotension + hyponatremia + hyperkalemia + AMS → immediate stress-dose steroids; never stop chronic glucocorticoids abruptly — taper slowly to avoid HPA-suppression crisis.
- Sheehan syndrome: postpartum hemorrhage + hypotension → pituitary infarct; earliest clue is failure of lactation, then gradual panhypopituitarism (amenorrhea, fatigue, cold intolerance).
- Bitemporal hemianopia + headache: think pituitary macroadenoma compressing optic chiasm; craniopharyngioma in a child or older adult (bimodal age, calcified + cystic, “machine oil” fluid).
- SIADH drug culprits: carbamazepine and oxcarbazepine are the high-yield AED causes; also SSRIs, antipsychotics, MDMA, opioids — check med list before workup.
- Kallmann syndrome: hypogonadotropic hypogonadism + anosmia — failed migration of GnRH neurons from olfactory placode; a classic boards-style pairing.
🔍 Quick ReferenceAnatomy / axes · Sodium / fluid disorders · Pituitary lesions
Anatomy / axes
- Supraoptic + paraventricular nuclei → ADH + oxytocin to posterior pituitary (neurohypophysis, neural ectoderm)
- Anterior pituitary from Rathke pouch (oral ectoderm) → TSH, ACTH, GH, FSH, LH, prolactin via portal system
- Dopamine tonic inhibition → prolactin (only AP hormone under tonic inhibition)
- Pulsatile GnRH → LH/FSH release; continuous GnRH (leuprolide) suppresses
- Suprachiasmatic nucleus + retinohypothalamic tract (melanopsin ipRGCs) → circadian pacemaker → pineal melatonin
- Arcuate POMC (anorexigenic) vs NPY/AgRP (orexigenic); leptin ↑POMC, ghrelin ↑AgRP → appetite regulation; setmelanotide (MC4R agonist) for genetic obesity
- OVLT + subfornical organ (circumventricular) → osmoreceptors driving thirst + ADH release
Sodium / fluid disorders
- Euvolemic hyponatremia + high urine osm + high urine Na + low BUN/uric acid → SIADH (fluid restrict ± tolvaptan/conivaptan)
- Hyponatremia + volume depletion + negative Na balance after SAH → cerebral salt wasting (replace Na + volume; fludrocortisone)
- Polyuria + polydipsia + hypernatremia + dilute urine, responds to DDAVP → central DI
- Polyuria on lithium / hypercalcemia / hypokalemia, no DDAVP response → nephrogenic DI
- Rapid Na correction → locked-in / quadriparesis days later → central pontine myelinolysis (osmotic demyelination)
- Hypotension + hyperkalemia + hyponatremia + AMS → acute adrenal insufficiency / Addisonian crisis
Pituitary lesions
- Bitemporal hemianopia + headache + hypopituitarism → pituitary macroadenoma compressing chiasm
- Sudden thunderclap headache + ophthalmoplegia + vision loss ± hypotension → pituitary apoplexy — stabilize + stress-dose hydrocortisone when adrenal insufficiency possible; urgent transsphenoidal decompression for worsening vision / reduced consciousness / severe neuro-ophthalmic compromise; stable patients may be observed
- Postpartum hemorrhage → failure of lactation → gradual panhypopituitarism → Sheehan syndrome
- PRL >200 ng/mL + amenorrhea/galactorrhea → prolactinoma (cabergoline > bromocriptine first-line); stalk effect typically <100–150 ng/mL; 100–200 is gray zone; in large macroadenomas with modest PRL, repeat PRL with dilution to exclude hook effect before labeling nonfunctioning
- Coarse features, enlarged hands/jaw, sweating, carpal tunnel, ↑IGF-1 → acromegaly (transsphenoidal ± octreotide/lanreotide; pegvisomant)
- Central obesity, striae, proximal weakness, ↑midnight cortisol + ACTH → Cushing disease (ACTH microadenoma; transsphenoidal; ketoconazole/metyrapone/mifepristone)
- Calcified + cystic suprasellar mass, “machine oil” fluid, bimodal age → craniopharyngioma
- Incidental CSF-filled sella in obese female, normal pituitary function → primary empty sella
- Anosmia + hypogonadotropic hypogonadism → Kallmann syndrome (failed GnRH neuron migration)
Hypothalamic-Pituitary Axis Overview
Functional Anatomy
- Hypothalamus — produces releasing and inhibiting hormones that regulate the anterior pituitary
- Hypothalamic-hypophyseal portal system — capillary network that carries hypothalamic hormones directly to anterior pituitary; bypasses systemic circulation
- Anterior pituitary (adenohypophysis) — derived from Rathke's pouch (oral ectoderm); synthesizes its own hormones under hypothalamic control
- Posterior pituitary (neurohypophysis) — derived from neural ectoderm; does NOT synthesize hormones; stores and releases ADH and oxytocin made in hypothalamic nuclei
- Pituitary stalk — connects hypothalamus to pituitary; stalk transection → loss of all anterior pituitary hormones EXCEPT prolactin (which increases due to loss of dopamine inhibition)
Key Hypothalamic Nuclei
| Nucleus | Hormone Produced | Key Function |
|---|---|---|
| Supraoptic nucleus | ADH (vasopressin) | Water balance; transported to posterior pituitary |
| Paraventricular nucleus | Oxytocin (+ some ADH); CRH | Milk letdown, uterine contractions; stress response |
| Arcuate nucleus | GHRH, dopamine (PIF), POMC (→ACTH/MSH/β-endorphin), NPY/AgRP | GH, prolactin inhibition, energy/appetite |
| Preoptic nucleus / medial preoptic area | GnRH (pulsatile) | GnRH release + thermoregulation (cooling/heat dissipation); GnRH neurons migrate from olfactory placode (Kallmann syndrome if KAL1/anosmin failure) |
| Suprachiasmatic nucleus | N/A (pacemaker) | Circadian rhythm; receives direct retinal input |
| Ventromedial nucleus | N/A | Satiety center; lesion → hyperphagia/obesity |
| Lateral hypothalamus | Orexin/hypocretin | Hunger, arousal; loss → narcolepsy type 1 |
| Anterior/preoptic hypothalamus | N/A | Heat dissipation center (sweating, vasodilation); lesion → hyperthermia |
| Posterior hypothalamus | N/A | Heat conservation/generation (shivering, vasoconstriction); lesion → poikilothermia/hypothermia |
Board Pearl
Pituitary stalk transection causes deficiency of all anterior pituitary hormones EXCEPT prolactin, which RISES. Prolactin is the only anterior pituitary hormone under tonic inhibitory control (by dopamine). Loss of dopamine → hyperprolactinemia.
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