Click a card to flip · 31 cards totalLOs 1–6 + Receptor Signaling Review
LO 1 · Anatomy Lecture Notes · In House Anki
Where is the pituitary gland located, what connects it to the hypothalamus, and through what two systems does the hypothalamus control each pituitary lobe?
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Answer
Location: Sella turcica of the sphenoid bone (brain base) Connection: Infundibulum (pituitary stalk)
Anterior pituitary (adenohypophysis): controlled by the hypophyseal portal system — releasing/inhibiting hormones reach it through a vascular route Posterior pituitary (neurohypophysis): controlled by direct axonal projections from hypothalamic neurons (ADH + oxytocin stored/released from nerve terminals)
LO 1 · Portal System Lecture Notes · In House Anki
Trace the route of a hypothalamic releasing hormone from secretion to its anterior pituitary target. Why does this system matter?
Why it matters: allows VERY HIGH local concentrations of tiny amounts of releasing hormone without systemic exposure. The median eminence is a circumventricular organ (no BBB) — essential for this direct hypothalamus → AP communication.
LO 2 · PVN / SON Lecture Notes · In House Anki
Which two hypothalamic nuclei make ADH and oxytocin, and how are the hormones transported?
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Answer
Supraoptic nucleus (SON): primarily ADH (+ some oxytocin) Paraventricular nucleus (PVN): primarily oxytocin (+ some ADH, also CRH & TRH for the AP)
Synthesized as preprohormones → packaged on neurophysin carriers → transported down axons → stored in Herring bodies at nerve terminals in posterior pituitary → released into fenestrated capillaries on stimulus.
Nicknamed the "nonapeptides" (9 amino acids each).
LO 2 · Hypothalamic Hormones Lecture Notes · In House Anki
List the seven classical hypothalamic hormones that control the anterior pituitary, each with its target hormone.
LO 3 · Pituitary Lobes Lecture Notes · In House Anki
What are the three embryologic/functional subdivisions of the pituitary, and their origins?
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Answer
Adenohypophysis (anterior, pars distalis): derived from Rathke's pouch (oral ectoderm) → glandular epithelial tissue Pars intermedia: also from Rathke's pouch; secretes MSH; mostly non-functional in adults Neurohypophysis (posterior, pars nervosa): derived from neuroectoderm of the diencephalon → true neural tissue
Clinical: Rathke's pouch remnants → craniopharyngioma (classic in children).
LO 3 · Histology Lecture Notes · In House Anki
How do you distinguish acidophils, basophils, and chromophobes on H&E, and which hormones do each secrete?
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Answer
Acidophils (eosin-pink): Somatotropes → GH; Lactotropes → PRL (2 of 6 anterior hormones)
LO 3 · Neurohypophysis Lecture Notes · In House Anki
What are Herring bodies and pituicytes, and what do they tell you about the posterior pituitary's embryology?
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Answer
Herring bodies: eosinophilic swellings at axon terminals of SON/PVN neurons; store ADH and oxytocin granules ready for release. Pituicytes: glial support cells of the posterior lobe (similar to astrocytes).
Both confirm the posterior pituitary is NEURAL tissue (diencephalic origin), not glandular. Contrast with adenohypophysis which has discrete secretory cells.
LO 4 · Hormone Map Lecture Notes · In House Anki
Name the six anterior pituitary hormones and classify each by cell type and peptide/glycoprotein chemistry.
Rule: the problem is where the hormone LEVEL DROPS first — everything UPSTREAM rises (attempting to compensate); everything DOWNSTREAM falls. "Primary" = farthest downstream. Apply the same logic to cortisol/ACTH and sex steroid axes.
LO 6 · Exogenous Hormone Lecture Notes · In House Anki
A patient on chronic oral prednisone — predict their CRH, ACTH, and endogenous cortisol levels. Why does sudden withdrawal cause crisis?
Serum total T may look normal (exogenous replaces endogenous) but LH/FSH and intratesticular testosterone are suppressed. HCG (LH-like) is used to preserve fertility.
LO 4 · Posterior Hormones Lecture Notes · In House Anki
What are the two physiologic stimuli for ADH release and the receptor/signaling used at its target?
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Answer
Stimuli: 1. ↑ plasma osmolarity (detected by osmoreceptors at OVLT — a circumventricular organ) 2. ↓ effective circulating volume (baroreceptor reflex)
Clinical applications: • Leuprolide (GnRH analog) — given CONTINUOUSLY suppresses HPG axis (prostate cancer, central precocious puberty, endometriosis) • Pulsatile leuprolide pump — restores fertility in hypothalamic hypogonadism
LO 1 · Circumventricular Organs Lecture Notes · In House Anki
What are the circumventricular organs (CVOs), and which four are functionally important in endocrinology?
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Answer
CVOs = brain regions near the ventricles where the blood-brain barrier is minimal or absent → allows direct exchange of hormones and peptides between blood and CNS.
Four endocrine-relevant CVOs: 1. Median eminence — first capillary plexus of the hypophyseal portal system; allows hypothalamic releasing hormones to enter portal blood 2. Neurohypophysis (posterior pituitary) — ADH/oxytocin released here into fenestrated capillaries 3. OVLT (organum vasculosum of the lamina terminalis) — senses blood OSMOLARITY to regulate ADH release 4. Pineal gland — secretes melatonin into blood
LO 2 · OVLT & ADH Lecture Notes · In House Anki
How does the OVLT sense plasma osmolarity, and what is its role in ADH (vasopressin) regulation?
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Answer
OVLT (organum vasculosum of the lamina terminalis) lies on the anterior wall of the third ventricle — a circumventricular organ WITHOUT a blood-brain barrier.
Specialized osmoreceptor neurons in the OVLT sense increased plasma tonicity (hyperosmolarity) and project to the supraoptic and paraventricular nuclei → trigger ADH release from the posterior pituitary.
Also stimulates thirst via projections to cortex.
This is the primary way the hypothalamus "tastes" the blood — the BBB shields most CNS neurons, but CVOs like the OVLT are specifically designed to sample plasma composition.
LO 2 · Dorsomedial Nucleus (GnIH) Lecture Notes · In House Anki
What hypothalamic hormone is produced by the dorsomedial nucleus, and what does it do?
GnIH is a small (8-AA) single-chain peptide that INHIBITS FSH and LH release from anterior pituitary gonadotropes — the counter-regulator to GnRH from the medial preoptic nucleus.
Part of the fine-tuning circuit that creates PULSATILE LH/FSH output. Similar "releaser-vs-inhibitor" pairings include GHRH/somatostatin for GH and, less strongly, TRH/somatostatin for TSH.
LO 1/4 · Direct SNS Pathway Lecture Notes · In House Anki
Beyond the two classical routes (portal system + posterior pituitary axons), how does the hypothalamus directly control the adrenal medulla?
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Answer
The hypothalamus has a THIRD output pathway: direct control of endocrine organs via the sympathetic nervous system.
Hypothalamic neurons → descend to spinal cord intermediolateral column → preganglionic sympathetic fibers → directly synapse on adrenal medulla chromaffin cells (NO postganglionic neuron — the adrenal medulla IS a modified sympathetic ganglion).
On activation, chromaffin cells release epinephrine (80%) + norepinephrine (20%) into blood within SECONDS — the fastest "endocrine" response in the body, enabling fight-or-flight.
LO 4 · Oxytocin Functions Lecture Notes · In House Anki
List the expanded functions of oxytocin beyond labor and lactation.
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Answer
Classical peripheral functions: • Uterine contractions during labor (positive feedback with cervical stretch — Ferguson reflex) • Milk let-down reflex in breastfeeding
Newer-recognized CNS/peripheral functions (as a neuropeptide): • Pair bonding / social recognition / trust • Memory (especially social memory) • Modulation of appetite and metabolism • Cardiovascular effects (mild ↓ BP) • Anxiolytic effects
Research interest: oxytocin as potential therapy for autism spectrum disorders, social-anxiety disorder, postpartum depression — though clinical efficacy remains unproven.
LO 4 · ADH Functions Lecture Notes · In House Anki
List the three major physiologic functions of ADH (vasopressin) and where each receptor subtype acts.
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Answer
1. Water retention — V2 receptor (Gs/cAMP) on renal collecting duct principal cells → ↑ aquaporin-2 insertion → water reabsorption. MAIN physiologic role.
2. Vasoconstriction — V1 receptor (Gq/IP3/Ca²⁺) on vascular smooth muscle → vasoconstriction at high ADH levels (e.g., hemorrhagic shock, "vasopressin" pressor therapy).
3. Anterior pituitary ACTH co-regulation — V1b/V3 receptor on corticotropes, synergistic with CRH.
Also contributes to body temperature regulation and peripheral vascular tone. Triggered by ↑ plasma osmolarity (OVLT) and ↓ effective circulating volume (baroreceptor reflex).
Receptor #1 · LO 4 · Gs Lecture Notes · In House Anki
Gs-coupled GPCRs — what second messenger? Name 5 hypothalamic / pituitary hormones that use Gs.
Laron syndrome: GH receptor mutation → no JAK2 recruitment → no IGF-1 → growth failure despite ↑↑GH.
Receptor #6 · LO 4 · Guanylyl Cyclase Lecture Notes · In House Anki
Which endocrine hormones signal through membrane guanylyl cyclase, and what is the second messenger?
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Answer
Membrane guanylyl cyclase → ↑cGMP → protein kinase G (PKG)
Hormones: ANP (atrial) and BNP (brain/ventricular) natriuretic peptides → released on atrial/ventricular stretch → natriuresis + vasodilation + ↓aldosterone + ↓renin.
Also: NO (nitric oxide) activates SOLUBLE (cytoplasmic) guanylyl cyclase — same downstream pathway, different receptor.
Clinical: BNP elevated in heart failure (diagnostic/prognostic marker).
Receptor #7 · LO 4/5 · Nuclear Receptor Superfamily Lecture Notes · In House Anki
What are the four major ligand classes for nuclear receptors?
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Answer
1. Steroid hormones — cortisol (GR), aldosterone (MR), estrogen (ER), progesterone (PR), testosterone/DHT (AR) 2. Thyroid hormone — T3 via TR (heterodimer with RXR) 3. Vitamin D — 1,25-(OH)₂-D via VDR (heterodimer with RXR) 4. Retinoic acid — via RAR (heterodimer with RXR)
All are ligand-gated transcription factors with DBD, LBD, TAD domains. Orphan receptor PPAR binds fatty acids + fibrates (α) / TZDs (γ). Relevant at the H-P axis: feedback of steroid/thyroid hormone back onto hypothalamus/pituitary via nuclear receptors.
hCG-TSH Cross-Reactivity · LO 4 (#17) Lecture Notes · In House Anki
Which subunit do hCG and TSH share, and what is the clinical consequence at high hCG?
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Answer
Both are glycoprotein hormones with an identical α-subunit; β-subunits are unique (β-hCG = pregnancy test marker). At massively elevated hCG levels (molar pregnancy, choriocarcinoma, hyperemesis gravidarum, hCG > ~100,000 mIU/mL), partial β-similarity allows hCG to cross-activate the TSH receptor → thyroid hormone release → hCG-mediated hyperthyroidism with suppressed TSH. Resolves after tumor evacuation.
Regulated vs Constitutive · LO 2/4 (#19) Lecture Notes · In House Anki
Distinguish REGULATED vs CONSTITUTIVE hormone secretion, and place each hypothalamic/pituitary hormone class into the correct bucket.
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Answer
REGULATED: hormone is pre-stored in secretory granules; released by Ca²⁺-triggered exocytosis on stimulus. Fast (seconds). • All anterior pituitary hormones (GH, PRL, ACTH, TSH, LH, FSH) — stored in secretory granules • Posterior pituitary hormones — ADH, oxytocin stored in Herring bodies • Hypothalamic releasing hormones — TRH, CRH, GnRH, GHRH (+ somatostatin, dopamine) stored at axon terminals • Adrenal medulla catecholamines in chromaffin granules
CONSTITUTIVE: no storage possible; hormone made de novo on demand and diffuses out immediately. • Adrenal cortex steroids (cortisol, aldosterone, DHEA) — made from cholesterol via StAR/desmolase when stimulated • Sex steroids (testosterone, estradiol)
Rule: lipid-soluble = can't be packaged in membrane granules = constitutive.
FIB 01 · LO 1 Lecture Notes · Study Guide
The pituitary gland sits in the _____________ of the sphenoid bone and is connected to the hypothalamus by the _____________ (pituitary stalk).
FIB 02 · LO 1 Lecture Notes · Study Guide
The anterior pituitary is reached by hypothalamic hormones through the _____________ system (vascular), whereas the posterior pituitary receives hormones via _____________ projections (neural) from the hypothalamus.
FIB 03 · LO 2 Lecture Notes · Study Guide
ADH and oxytocin (the "_____________peptides") are synthesized in the _____________ and _____________ nuclei.
FIB 04 · LO 3 Lecture Notes · Study Guide
The adenohypophysis is derived from _____________ (a pouch of oral ectoderm), while the neurohypophysis is derived from _____________.
Hint: Rathke's remnants can form craniopharyngiomas.
FIB 05 · LO 3 Lecture Notes · Study Guide
Two of the six anterior pituitary hormones come from _____________ cells (GH, prolactin), while four come from _____________ cells (FSH, LH, ACTH, TSH — mnemonic "B-FLAT").
FIB 06 · LO 4 Lecture Notes · Study Guide
Prolactin is uniquely under _____________ inhibition by _____________ (released by arcuate-nucleus neurons). Drugs that block this neurotransmitter's D2 receptors cause _____________.
FIB 07 · LO 4 Lecture Notes · Study Guide
TRH primarily stimulates TSH release but ALSO raises _____________, which is why primary hypothyroidism (high TRH) can present with galactorrhea. GHRH stimulates GH; somatostatin inhibits both GH and _____________.
FIB 08 · LO 5 Lecture Notes · Study Guide
In the cortisol axis: cortisol exerts _____________-loop negative feedback on CRH and ACTH, ACTH exerts _____________-loop feedback on CRH, and CRH can inhibit its own release via _____________-loop feedback.
FIB 09 · LO 5 Lecture Notes · Study Guide
The LH surge at mid-cycle is driven by a switch from negative to _____________ feedback by _____________ on GnRH release.
FIB 10 · LO 6 Lecture Notes · Study Guide
In secondary hypothyroidism (pituitary lesion): T4 is _____________, TSH is _____________, and TRH is _____________.
Hint: in secondary, the problem is the pituitary; everything UPSTREAM compensates.
FIB 11 · LO 1 Lecture Notes · Study Guide
Four circumventricular organs have endocrine relevance because they LACK a blood-brain barrier: the _____________ eminence (site of portal capillary plexus), the posterior pituitary, the _____________ (osmoreceptor for ADH), and the _____________ gland (secretes melatonin).
FIB 12 · LO 1/4 Lecture Notes · Study Guide
Beyond portal and axonal routes, a third hypothalamic output pathway is the _____________ nervous system, which directly innervates chromaffin cells of the adrenal _____________ to release _____________ (80%) and NE (20%) into the blood within seconds.
Hint: the adrenal medulla = modified sympathetic ganglion.
Instructions: Drag and drop the items into the correct order, then click "Check Order." Use "Show Answer" if you get stuck.
Drill 1 — Hypophyseal Portal System: Hormone to Target Lecture Notes · Study Guide
Arrange the steps from hypothalamic neuron to anterior pituitary hormone release.
⠿Hormone enters hypophyseal portal veins
⠿Hypothalamic neuron secretes releasing hormone
⠿Anterior pituitary cell releases tropic hormone into systemic circulation
⠿Hormone enters first capillary plexus in the median eminence
⠿Hormone reaches second capillary plexus in anterior pituitary and binds target receptor
Hypothalamic neuron secretes releasing hormone
First capillary plexus in median eminence (no BBB)
Portal veins carry hormone down the stalk
Second capillary plexus in anterior pituitary — binds cell-surface receptors
Anterior pituitary hormone released into systemic circulation
First capillary plexus of hypophyseal portal system (hypothalamic releasers enter)
Axon terminals from SON/PVN release ADH + oxytocin (Herring bodies)
Score: 0/4 correct
If-Then Cascade Drills: Ten cascades aligned to four reference tables — Primary/Secondary/Tertiary Rule, Hypothalamic Nuclei & Their Hormones, Anterior Pituitary Cell/Control/Action, and Feedback Loop Hierarchy. Click "Predict →" at each step to rehearse your axis reasoning from anatomy → hormone → lab → clinical.
Cascade 1 · LO 6 — Primary / Secondary / Tertiary: Hypothyroidism Workup Lecture Notes · Study Guide
Trigger
Patient with fatigue, cold intolerance, weight gain. Free T4 is LOW. Localize the lesion using the Primary/Secondary/Tertiary rule.
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Step 1 Rule of tiers
When terminal hormone (T4) is LOW, look at the trophic hormone. HIGH TSH = pituitary working fine + sensing deficit = PRIMARY (target organ failure). LOW/inappropriately-normal TSH = pituitary can't respond = SECONDARY (pituitary) or TERTIARY (hypothalamus). TRH stimulation test distinguishes secondary from tertiary.
Pattern: ↓T4, ↓ or inappropriately-normal TSH, ↑TRH. Causes: pituitary adenoma, post-surgical/radiation, Sheehan, empty sella. TRH stimulation test: TSH fails to rise (pituitary can't respond).
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Step 4 Tertiary (hypothalamic)
Pattern: ↓T4, ↓TSH, ↓TRH. Causes: craniopharyngioma, hypothalamic irradiation, infiltrative disease. TRH stimulation test: TSH DOES rise (pituitary is intact — it just wasn't being stimulated). Treatment: levothyroxine; monitor free T4 (NOT TSH — unreliable in central disease).
Cascade 2 · LO 6 — Primary / Secondary / Tertiary: Hypercortisolism Workup Lecture Notes · Study Guide
Trigger
Cushingoid patient — moon facies, central obesity, striae. 24-hr urine FREE cortisol is HIGH. Localize the source.
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Step 1 Rule of tiers
Terminal hormone (cortisol) HIGH → look at ACTH. LOW ACTH = adrenal autonomous source = PRIMARY (cortex-level). HIGH ACTH = something driving the pituitary or ectopic = SECONDARY (pituitary Cushing's DISEASE) or ECTOPIC (non-pituitary tumor, e.g., SCLC). High-dose dex suppression test distinguishes pituitary vs ectopic.
Pattern: ↑cortisol, ↑ACTH, ↓CRH. Corticotroph adenoma (Cushing's DISEASE) — most common cause of endogenous Cushing's. Partial feedback: responds to HIGH-dose dex suppression (SUPPRESSES cortisol).
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Step 4 Ectopic (non-pituitary)
Pattern: ↑cortisol, ↑↑ACTH, ↓CRH. SCLC, bronchial carcinoid, thymic carcinoid. Tumor ACTH is autonomous — does NOT suppress with high-dose dex. Hyperpigmentation (ACTH shares POMC with α-MSH); severe hypokalemic metabolic alkalosis (cortisol overwhelms 11β-HSD2 → spills onto MR).
Cascade 3 · LO 6 — Primary / Secondary / Tertiary: Hypogonadism Workup Lecture Notes · Study Guide
Trigger
35-year-old man with low testosterone, low libido, infertility, loss of morning erections. Localize the lesion.
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Step 1 Rule of tiers
Terminal hormone (testosterone) LOW → look at LH/FSH. HIGH LH/FSH = HYPERgonadotropic hypogonadism = PRIMARY (testicular failure). LOW LH/FSH = HYPOgonadotropic hypogonadism = SECONDARY (pituitary) or TERTIARY (hypothalamic). GnRH stimulation test distinguishes 2° vs 3°.
Trigger
Child with short stature, delayed bone age, growth velocity < 5 cm/year. Workup is pointing toward GH deficiency. Localize the lesion.
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Step 1 Rule of tiers (GH axis)
The GH axis has a twist: GH itself is pulsatile and unreliable as a random measurement. Use IGF-1 (terminal mediator) as the stable marker. Low IGF-1 → confirm with GH stimulation tests (insulin tolerance test, arginine). Then localize: primary = liver/target tissue resistance (Laron); secondary = pituitary; tertiary = hypothalamus.
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Step 2 Primary (end-organ)
Laron syndrome (GH receptor mutation) → liver cannot respond to GH. Pattern: ↓IGF-1, ↑↑GH, ↑GHRH. Severe short stature. Treat with recombinant IGF-1 (mecasermin) — NOT GH.
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Step 3 Secondary (pituitary)
Pattern: ↓IGF-1, ↓GH, ↑GHRH. Pituitary adenoma (mass effect), post-surgical, radiation, congenital isolated GH deficiency. GH fails to rise with stimulation (insulin tolerance test or arginine). Treat with recombinant GH (somatropin).
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Step 4 Tertiary (hypothalamic)
Pattern: ↓IGF-1, ↓GH, ↓GHRH. Hypothalamic tumor (craniopharyngioma is classic cause of pediatric short stature), irradiation, infiltrative. GHRH stim test: GH DOES rise (pituitary is responsive — just wasn't being signaled). Treat with recombinant GH.
Cascade 5 · LO 2 — Hypothalamic Nuclei: OVLT → SON/PVN → ADH Release Lecture Notes · Study Guide
Trigger
Dehydrated patient: serum osmolality 305 mOsm/kg (normal 275–295). Walk through the specific hypothalamic nuclei involved.
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Step 1 Circumventricular osmosensor
The OVLT (organum vasculosum of the lamina terminalis) sits on the anterior 3rd-ventricle wall — a CVO without BBB. Its osmoreceptor neurons shrink in hypertonic plasma → depolarize → activate downstream neurons. The subfornical organ (SFO) also senses angiotensin II and reinforces the thirst/ADH response.
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Step 2 Hypothalamic nuclei
OVLT projects to supraoptic nucleus (SON) and paraventricular nucleus (PVN). Magnocellular neurons there synthesize ADH (nonapeptide, ~9 AA) packaged with its carrier neurophysin II. Parallel signaling goes to cortex for conscious thirst.
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Step 3 Axonal transport + release
ADH and carrier travel by fast axonal transport down the infundibulum to the posterior pituitary (neurohypophysis), stored in Herring bodies near pituicyte cell bodies, then released directly into systemic circulation when SON/PVN are depolarized. This is different from the portal-system route of anterior-pituitary releasing hormones.
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Step 4 Kidney + clinical
ADH → V2 receptor (Gs-coupled) on collecting duct principal cells → ↑cAMP/PKA → aquaporin-2 insertion → water reabsorption → urine osmolality rises > 600 mOsm/kg. Failure at SON/PVN = central DI; failure at V2/aquaporin = nephrogenic DI. Desmopressin (DDAVP) test discriminates.
Cascade 6 · LO 2/4 — Hypothalamic Nuclei: Arcuate Nucleus Dopamine → Lactotroph Inhibition Lecture Notes · Study Guide
Trigger
Schizophrenic patient started on risperidone (typical/atypical antipsychotic with D2 receptor antagonism). Presents with galactorrhea + amenorrhea 2 months later.
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Step 1 Normal tonic inhibition
Unlike other anterior pituitary hormones, prolactin is under tonic INHIBITION. The inhibitor is dopamine (a.k.a. PRL-inhibiting hormone, PIH) synthesized in the arcuate (infundibular) nucleus of the hypothalamus and secreted into the hypophyseal portal system. Dopamine binds D2 receptors on lactotrophs → Gi → ↓cAMP → ↓PRL release.
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Step 2 D2 blockade
Antipsychotics block D2 receptors centrally (intended CNS action) AND at the lactotroph (unintended side effect). With D2 blocked at lactotroph, dopamine cannot inhibit → disinhibited PRL secretion → hyperprolactinemia. Same effect for any D2 antagonist: metoclopramide (antiemetic), prochlorperazine, haloperidol.
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Step 3 Downstream on GnRH
High PRL suppresses pulsatile GnRH from another arcuate subpopulation → ↓ LH/FSH → hypogonadotropic hypogonadism. In women: amenorrhea, infertility, ↓ estradiol → osteoporosis risk. In men: ↓ libido, erectile dysfunction, gynecomastia.
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Step 4 Labs + Treatment
↑ PRL (usually < 100 ng/mL for drug-induced vs > 200 for prolactinoma), ↓ LH/FSH, ↓ estradiol/testosterone. Pituitary MRI negative. Treatment: stop the offending drug if possible; switch to an antipsychotic with minimal D2 blockade at pituitary (aripiprazole, quetiapine). This cascade also explains why DOPAMINE AGONISTS (cabergoline, bromocriptine) treat prolactinoma — they mimic the missing inhibitory signal.
Trigger
Cold, fatigued patient. Trace the thyrotroph axis from hypothalamus → pituitary cell → thyroid → target tissues, including the specific histologic cell type involved.
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Step 1 Hypothalamus
TRH (thyrotropin-releasing hormone, tripeptide) is produced by paraventricular nucleus (PVN) neurons. TRH travels via the hypophyseal portal system from the median eminence to the anterior pituitary. TRH also stimulates prolactin secretion (explains hyperprolactinemia sometimes seen in hypothyroidism).
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Step 2 Pituitary cell type
TRH binds Gq-coupled TRH receptor on thyrotrophs — basophilic cells (H&E) located predominantly in the anteromedial portion of the anterior pituitary. Thyrotrophs are ~5% of pituitary cells. TSH (glycoprotein, shares common α with LH/FSH/hCG; unique β-subunit) is secreted into systemic circulation.
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Step 3 Thyroid follicle action
TSH binds Gs-coupled TSH receptor on thyroid follicular cell basolateral membrane → ↑cAMP → stimulates ALL steps of thyroid hormone synthesis: Na/I symporter activity, thyroglobulin iodination by TPO, endocytosis of colloid, proteolysis of thyroglobulin → release of T4 (~90%) and T3 (~10%). Long-term: follicular hyperplasia + goiter formation.
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Step 4 Target tissue + feedback
T4 converted peripherally to active T3 by deiodinases. T3 binds nuclear thyroid receptor (TR) → heterodimerizes with RXR on thyroid response elements → transcription of genes increasing basal metabolic rate, cardiac output, thermogenesis. Feedback (LONG LOOP): T4/T3 inhibit both thyrotroph TSH release AND hypothalamic TRH release. Short loop: TSH on hypothalamic TRH neurons.
Trigger
Trace the somatotroph axis: which cell type makes GH, which hypothalamic signals regulate it, and how does GH's action reach target tissues?
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Step 1 Hypothalamus (dual control)
GH secretion is tightly controlled by TWO opposing hypothalamic signals: GHRH (from arcuate nucleus, stimulates) and somatostatin (SRIF) (from periventricular/arcuate, inhibits). Ghrelin from the stomach also stimulates GH via a separate receptor (GHS-R). Pulsatile GH release (every ~4 hours, largest pulse during sleep) is driven by reciprocal GHRH/SRIF rhythms.
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Step 2 Pituitary cell type
Somatotrophs are acidophilic cells (H&E) — pink-staining, ~50% of anterior pituitary cells (most abundant!). GHRH binds Gs-coupled GHRH receptor → ↑cAMP → secretes GH (peptide, 191 AA, same family as prolactin and hPL). Somatostatin binds Gi on somatotrophs → ↓cAMP → inhibits.
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Step 3 Liver + peripheral action
GH binds hepatic GH receptor — a cytokine receptor using JAK2/STAT5 signaling → ↑ transcription of IGF-1 (main growth-promoting signal). IGF-1 travels in plasma bound to IGFBP-3 (long half-life) and drives bone/cartilage/muscle growth. Direct GH effects on target tissues: diabetogenic (↑ hepatic gluconeogenesis, ↓ muscle glucose uptake), lipolytic.
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Step 4 Feedback loops
Long loop: IGF-1 inhibits somatotroph GH release AND stimulates hypothalamic somatostatin → compound suppression. Short loop: GH itself stimulates somatostatin release from hypothalamus. Ultrashort loop: somatostatin inhibits its own release (autoreceptors). This creates crisp pulsatile GH — rising GHRH pulses are sharply terminated by somatostatin feedback.
Cascade 9 · LO 5 — Feedback Loop Hierarchy: Long Loop (Cortisol → HPA Axis) Lecture Notes · Study Guide
Trigger
Patient takes supraphysiologic exogenous prednisone for 6 months. Trace the LONG-loop feedback through the HPA axis.
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Step 1 Long loop (classical)
Long loop = terminal effector hormone feeds back on both pituitary AND hypothalamus. Cortisol (exogenous or endogenous) crosses BBB and directly inhibits: (1) corticotrophs — ↓ ACTH release; (2) paraventricular CRH neurons — ↓ CRH release. This is the DOMINANT regulatory loop for most axes (HPA, HPT, HPG, GH/IGF-1).
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Step 2 Short loop
Short loop = pituitary hormone feeds back on hypothalamus. ACTH itself (when surging) suppresses hypothalamic CRH. Adds a second tier of control that is faster than the long loop (doesn't require adrenal response). Observed for ACTH → CRH, LH → GnRH, TSH → TRH.
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Step 3 Ultrashort loop
Ultrashort loop = hypothalamic hormone inhibits its own release via autoreceptors on the same neurons or immediate neighbors. CRH neurons autoinhibit at high CRH output. GnRH and somatostatin neurons also show ultrashort feedback. This sharpens pulsatile hormone release.
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Step 4 Clinical: withdrawal risk
Chronic prednisone → long-loop suppression of BOTH CRH and ACTH → prolonged ↓ endogenous ACTH → adrenal zona fasciculata ATROPHIES. If prednisone is stopped abruptly, the atrophied adrenal cannot produce cortisol fast enough → iatrogenic adrenal crisis. Always TAPER steroids over weeks. Stress-dose steroids for surgery/illness are required for months after withdrawal. The practical consequence of chronic long-loop feedback suppression.
Cascade 10 · LO 5/6 — Feedback Loop Hierarchy: Positive Feedback Exception — LH Surge Lecture Notes · Study Guide
Trigger
Mid-menstrual-cycle ovulation. Most feedback is negative — but here's the one classical positive-feedback loop in the H-P system.
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Step 1 Normal negative feedback
Through most of the menstrual cycle, estradiol and progesterone exert classical NEGATIVE feedback on hypothalamus (↓ GnRH) and pituitary (↓ LH/FSH). Low-level estradiol damps GnRH pulses. Standard long-loop pattern.
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Step 2 Threshold switch
When the dominant follicle reaches maturity, estradiol rises rapidly above a threshold (~200 pg/mL for ≥ 48 hr). At this threshold, estradiol's feedback on pituitary gonadotrophs FLIPS from negative to POSITIVE — sensitizes gonadotrophs to GnRH, amplifies LH release dramatically. This is a rare true positive feedback loop in endocrinology.
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Step 3 LH surge
LH surge: LH rises 10-fold within 24–36 hours. The surge triggers ovulation (~36 hr after surge onset). FSH rises modestly too (smaller surge). This positive-feedback burst is the endpoint of the entire follicular phase.
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Step 4 Return to negative
After ovulation, corpus luteum secretes progesterone + estradiol. Progesterone RESTORES negative feedback on hypothalamus/pituitary → suppresses GnRH/LH/FSH for the luteal phase. If no pregnancy, corpus luteum regresses → progesterone falls → menses + disinhibited FSH rises again to begin the next cycle. Clinical relevance: OCPs supply constant exogenous estradiol + progesterone → blunt the LH surge → prevent ovulation. The other classical positive-feedback loop is oxytocin in labor (stretch → more OT → more contractions → more stretch).
Test Trap Mode: Each statement is TRUE or FALSE as written. Many reverse or mix facts that trip students up.
Trap 01 · LO 1 Lecture Notes
"The anterior pituitary is neural tissue derived from the diencephalon and is connected to the hypothalamus by direct axonal projections."
FALSE. Anterior pituitary = glandular epithelium from Rathke's pouch (oral ectoderm) — connected via the hypophyseal portal system (vascular). The POSTERIOR pituitary is neural tissue from diencephalon with direct axonal projections. Reversed fact pattern.
Trap 02 · LO 2 Lecture Notes
"ADH and oxytocin are synthesized by the cell bodies of the posterior pituitary and stored in Herring bodies."
FALSE. ADH and oxytocin are synthesized in the hypothalamus (cell bodies in SON and PVN). They are TRANSPORTED down axons to the posterior pituitary, where Herring bodies (axon terminal swellings) store them for release. The posterior pituitary has NO secretory cell bodies — only terminals and pituicytes (glia).
Trap 03 · LO 3 Lecture Notes
"Acidophils of the anterior pituitary release thyroid-stimulating hormone and adrenocorticotropic hormone."
FALSE. Acidophils release GH and prolactin (only 2 hormones). TSH and ACTH come from basophils (along with FSH and LH). Mnemonic: "B-FLAT" — Basophils: FSH, LH, ACTH, TSH.
Trap 04 · LO 4 Lecture Notes
"Cutting the pituitary stalk causes a fall in all six anterior pituitary hormones, including prolactin."
FALSE.Prolactin RISES after stalk transection. Prolactin is the ONLY AP hormone under TONIC INHIBITION (dopamine/PIH from arcuate nucleus). Remove dopamine delivery → lactotropes unleashed → ↑PRL. All five others fall (loss of their releasing hormones). Classic boards fact.
Trap 05 · LO 4 Lecture Notes
"Continuous, non-pulsatile GnRH infusion is the best way to restore fertility in a patient with hypothalamic amenorrhea."
FALSE. Continuous GnRH exposure DOWN-REGULATES pituitary GnRH receptors → paradoxical suppression of LH/FSH ("medical castration"). This is the mechanism of leuprolide for prostate cancer and endometriosis. PULSATILE GnRH (every 60–90 min via pump) restores normal gonadotropin rhythm and fertility.
Trap 06 · LO 5 Lecture Notes
"Ultra-short loop feedback refers to target-organ hormones inhibiting hypothalamic release."
FALSE. Ultra-short = a hypothalamic hormone inhibiting ITS OWN release via axon collaterals (e.g., CRH → ↓CRH). Target-organ → hypothalamus is the LONG loop. Ultra-short generates pulsatility and prevents receptor desensitization.
Trap 07 · LO 6 Lecture Notes
"A patient with primary hypothyroidism will have low T4 AND low TSH."
FALSE. In PRIMARY hypothyroidism, the thyroid gland is the problem → T4 falls → loss of long-loop negative feedback → TSH RISES (and TRH rises). Low T4 + low TSH indicates secondary (pituitary) or tertiary (hypothalamic) failure.
Trap 08 · LO 6 Lecture Notes
"A patient on chronic prednisone has suppressed endogenous cortisol but normal ACTH and CRH because the pituitary doesn't sense synthetic steroids."
FALSE. Exogenous glucocorticoids bind the SAME glucocorticoid receptor → full long-loop suppression of BOTH CRH and ACTH. Endogenous cortisol falls, adrenal atrophies. This is why steroids must be TAPERED — sudden cessation can precipitate adrenal crisis.
Trap 09 · LO 1 Lecture Notes
"The entire hypothalamus lacks a blood-brain barrier, which is why hypothalamic neurons can directly sense circulating hormones like insulin and leptin."
FALSE. Most of the hypothalamus HAS a normal BBB. Only SPECIFIC regions called circumventricular organs (CVOs) lack the BBB — including the median eminence, neurohypophysis, OVLT, subfornical organ, pineal gland, and area postrema. These specialized regions are the brain's dedicated "plasma-tasting" sites, which is why they can detect osmolarity, angiotensin, hormones, and drugs directly from the blood.
Trap 10 · LO 4 Lecture Notes
"The adrenal medulla is controlled by ACTH from the anterior pituitary, like the adrenal cortex."
FALSE. The adrenal MEDULLA is a modified sympathetic ganglion — controlled by direct preganglionic sympathetic innervation from the spinal cord (originating from hypothalamic autonomic output). ACTH controls the adrenal CORTEX (cortisol, aldosterone, androgens). This is why pituitary lesions cause cortisol deficiency but NOT catecholamine deficiency — and why Addison's (adrenal cortex failure) does not directly affect epinephrine production.
Reference Tab MCQs: Five Step 1-style questions testing the Reference tables directly. Select the best answer — the explanation reveals automatically.
Question 1 of 5 · LO 2 — Hypothalamic Nucleus → Hormone Lecture Notes · Study Guide
Per the "Hypothalamic Nuclei & Their Hormones" reference table, a lesion specifically affecting the supraoptic nucleus (SON) would most severely impair secretion of which hormone?
AOxytocin (primary source)
BADH / vasopressin (primary source)
CCRH
DDopamine (PIH)
ESomatostatin
B. Per the table, SON is the primary source of ADH (with some oxytocin). PVN (A) is primarily oxytocin (also CRH, TRH). CRH is from PVN (parvocellular part). Dopamine (D) is from arcuate nucleus. Somatostatin (E) is from periventricular nucleus. SON-specific injury → central DI without affecting labor/lactation as severely.
Question 2 of 5 · LO 4 — Pituitary Cell Identification Lecture Notes · Study Guide
Per the "Anterior Pituitary Hormones" reference table, a pituitary tumor biopsy shows acidophilic (eosin-pink) cells. Which TWO hormones are the most likely products?
AACTH and TSH
BFSH and LH
CGH and prolactin
DADH and oxytocin
EACTH and prolactin
C. Acidophils = somatotropes (GH) + lactotropes (prolactin). Basophils ("B-FLAT") = FSH, LH, ACTH, TSH. ADH/oxytocin (D) are posterior pituitary (neurohypophysis) — not made by adenohypophyseal cells. Acidophilic tumors are among the most common pituitary adenomas (prolactinoma ~50%, GH-secreting ~15%).
Question 3 of 5 · LO 5 — Feedback Hierarchy Lecture Notes · Study Guide
Per the "Feedback Loop Hierarchy" reference table, which of the following is the BEST example of ULTRA-SHORT loop feedback?
ACortisol inhibits ACTH and CRH
BACTH inhibits CRH at the hypothalamus
CCRH inhibits its own release via axon collaterals in the hypothalamus
C. Per the table, ultra-short loop = hypothalamic hormone inhibits ITSELF via axon collaterals. This creates pulsatile release (prevents receptor desensitization at the pituitary). A and E = long loop (target-organ hormone or its analog acts on hypothalamus+pituitary). B = short loop (pituitary hormone → hypothalamus). D = positive feedback.
Question 4 of 5 · LO 6 — Primary / Secondary / Tertiary Lecture Notes · Study Guide
Per the "Primary / Secondary / Tertiary Rule" reference table, a patient has cortisol 3 μg/dL (low), ACTH < 5 pg/mL (very low), and CRH fails to rise after insulin-induced hypoglycemia. Which level of the axis is failing?
APrimary adrenal insufficiency (Addison's)
BSecondary (pituitary) adrenal insufficiency
CTertiary (hypothalamic) adrenal insufficiency
DExogenous glucocorticoid excess
ECushing syndrome
C — Tertiary. Per the rule: the problem is where the hormone LEVEL DROPS FIRST; everything downstream falls. Here cortisol ↓, ACTH ↓, AND CRH is unresponsive → the hypothalamus is the primary site of failure (tertiary). Primary (A) would have ↑↑ACTH; secondary (B) would have normal/↑ CRH but failure of pituitary ACTH response. Exogenous steroid (D) would suppress via long-loop feedback but CRH would still be detectable and responsive on withdrawal.
Question 5 of 5 · LO 1/4 — Three Hypothalamic Output Pathways Lecture Notes · Study Guide
Per the "Three Hypothalamic Output Pathways" reference table, which pathway enables the fastest endocrine response to an acute stressor (seconds, not minutes)?
AHypophyseal portal system to anterior pituitary
BDirect axonal projection to posterior pituitary
CSympathetic preganglionic fibers to adrenal medulla chromaffin cells
DRetrograde transport to the limbic system
ECSF-mediated delivery via the choroid plexus
C. The sympathetic pathway is unique among hypothalamic outputs: preganglionic fibers synapse directly onto adrenal medulla chromaffin cells (a modified sympathetic ganglion — no postganglionic neuron). ACh triggers exocytosis of preformed epinephrine (80%) + NE (20%) within seconds → "fight-or-flight." Portal system (A) and posterior-pituitary pathway (B) are also rapid but rely on cell synthesis/vesicular release; seconds-scale catecholamine dumping is the fastest endocrine reflex in the body.
Hypothalamic Nuclei & Their Hormones
Nucleus
Hormone(s)
Target / Function
Supraoptic (SON)
ADH (primary), oxytocin
Posterior pituitary via axonal projection
Paraventricular (PVN)
Oxytocin (primary), ADH, CRH, TRH
Post. pituitary + anterior pituitary (CRH/TRH via portal)
Read it like a thermostat: wherever the DYSFUNCTION is, every step UPSTREAM tries to compensate (rises in hypofunction, falls in hyperfunction); everything DOWNSTREAM of the problem falls/rises with it.
Circumventricular Organs (CVOs) — Regions Without Blood-Brain Barrier
CVO
Endocrine Function
Clinical Relevance
Median eminence
First capillary plexus of hypophyseal portal system
Route for ALL hypothalamic releasing/inhibiting hormones to reach anterior pituitary
Neurohypophysis
Release site of ADH & oxytocin into fenestrated capillaries
Targeted by intranasal desmopressin (crosses thanks to no BBB)
OVLT
Osmoreceptor → triggers ADH + thirst
Senses hypernatremia; dysfunctional in adipsic DI
Subfornical organ (SFO)
Senses angiotensin II → thirst + ADH
Paired with OVLT in volume/osmolar sensing
Pineal gland
Secretes melatonin into blood (circadian rhythm)
Lesions → sleep/wake disturbances; melatonin supplements work systemically
Area postrema
Chemoreceptor trigger zone (CTZ) — emesis
Target of ondansetron (5-HT3), metoclopramide (D2), aprepitant (NK-1)
Why they matter: Because CVOs lack a BBB, circulating hormones (ADH, Ang II, leptin) and drugs (desmopressin, anti-emetics acting at the CTZ) can act directly on CNS tissue without crossing tight junctions. They are the brain's dedicated "plasma-sampling" sites.
How to use this map: each row pairs a Hub 1 core concept with its downstream Step 1 expansion — follow a concept left → right to anticipate USMLE integrations that appear in Hub 2.