Built from Endocrine Lecture 04 (LOs 1–6). Close notes. Fill from memory. Reveal only after a genuine attempt.

LO 1

ON EXAM — ALL steroids share the cyclopentanoperhydrophenanthrene 4-ring nucleus (rings A, B, C + 5-member D). Cholesterol = C27. Classes by carbon number: C21 PREGNANES (progestins, gluco, mineralo); C19 ANDROSTANES (androgens); C18 ESTRANES (estrogens, aromatic A ring). α = below plane, β = above plane. Key suffixes: -one = keto, -ol = hydroxyl, Δ or ene = double bond. Cortisol = 11β-OH, 17α-OH, 21-OH, 3-oxo-Δ⁴. Aldosterone = distinctive 18-aldehyde

The cholesterol backbone — shared 4-ring nucleus (LO 1)

Name of the shared 4-ring nucleus

Number of rings + ring shapes (6/5/6/5?)

Labels of the four rings

Total carbons in cholesterol

Why is the D-ring different from A/B/C?

Direction of carbon numbering (start where?)

Steroid classes by carbon number (LO 1)

Class (name)	Carbon count	Loss relative to cholesterol	Hormone examples	Signature structural feature
Pregnane
Androstane
Estrane

Nomenclature conventions (LO 1)

Suffix -one means…

Suffix -ol means…

Prefix Δⁿ or -ene means…

α vs β orientation — what do they mean?

Why is the estrogen A-ring "aromatic"?

Why is it called 11β-hydroxylase (not 11α)?

Reading key hormone names (LO 1)

Hormone	Carbon class	Key modifications (functional groups + positions)	Signature feature vs precursors
Cortisol
Aldosterone
Progesterone
DHEA
Testosterone
17β-Estradiol

LO 2

ON EXAM — ALL steroidogenesis shares: (1) cholesterol sourcing (LDL/SR-BI/de novo/lipid droplets mobilized by HSL under PKA); (2) StAR shuttles cholesterol OMM → IMM (ACUTE regulated rate-limiting step); (3) CYP11A1 (P450scc/desmolase) cleaves cholesterol → PREGNENOLONE on IMM (first committed step); (4) adrenodoxin reductase → adrenodoxin → CYP11A1 electron shuttle; (5) pregnenolone exits mitochondria for further modification in smooth ER (CYP17, CYP21) with some steps back in mitochondria (CYP11B1/B2); (6) 3β-HSD converts Δ⁵-steroids (pregnenolone, DHEA, 17-OH-pregnenolone) to Δ⁴-steroids (progesterone, androstenedione, 17-OH-progesterone)

Cholesterol sources for adrenal cortex (LO 2)

Source	Transporter / mechanism	Relative contribution (humans)
Circulating LDL
Circulating HDL
De novo synthesis
Lipid droplet reserve

Intracellular cholesteryl esters are hydrolyzed by Cholesteryl Ester Hydrolase (CEH / HSL) under PKA phosphorylation (downstream of ACTH → cAMP). This provides a rapid-release reserve.

The initial step — two sub-steps (LO 2)

Sub-step	Protein / enzyme	Reaction	Subcellular site	Regulation
1. Shuttle (acute rate-limiting)
2. Cleavage (first committed)

StAR full name

CYP11A1 aliases (three)

Cofactors needed by CYP11A1

Electron-donor shuttle feeding mitochondrial CYPs

ACTH cascade that rapidly induces StAR

Congenital lipoid adrenal hyperplasia — which protein mutated?

After pregnenolone — next shared step (LO 2)

3β-HSD substrate → product (name 3 examples)

3β-HSD subcellular location

Δ⁵ vs Δ⁴ steroid — why the distinction matters

Where does pregnenolone travel after its synthesis in mitochondria? (next compartment)

Shuttle logic of steroidogenic enzymes (LO 2)

Enzymes in MITOCHONDRIA (list 3)

Enzymes in SMOOTH ER (list 3)

Why "commuter" pathway — what does the steroid intermediate do between compartments?

LO 3

ON EXAM — 3 CORTICAL zones (outer → inner: GFR): Glomerulosa (salt — aldosterone only), Fasciculata (sugar — cortisol), Reticularis (sex — DHEA). PLUS adrenal medulla (neural crest → epinephrine). Regulation: Ang II + K⁺ drive glomerulosa; ACTH drives fasciculata + reticularis (NOT glomerulosa). Enzyme ownership: aldosterone synthase (CYP11B2) ONLY in glomerulosa; 17α-hydroxylase (CYP17) NOT in glomerulosa → explains why it can't make cortisol or androgens

Adrenal anatomy — zones + medulla (LO 3)

Layer (outer → inner)	Germ layer	Hormone class	Specific hormone	Main regulator	Clinical mnemonic
Zona glomerulosa
Zona fasciculata
Zona reticularis
Adrenal medulla

MNEMONIC: "Salt, Sugar, Sex, Epinephrine" — Glomerulosa (salt/aldo), Fasciculata (sugar/cortisol), Reticularis (sex/DHEA), Medulla (epi). "The deeper you go, the sweeter it gets."

Zone-specific enzyme ownership (LO 3)

Enzyme (gene)	Glomerulosa?	Fasciculata?	Reticularis?	What it does
CYP11A1 (P450scc / desmolase)
3β-HSD
CYP21A2 (21-hydroxylase)
CYP11B1 (11β-hydroxylase)
CYP11B2 (aldosterone synthase)
CYP17 (17α-hydroxylase)
CYP17 (17,20-lyase activity)

Why regulation differs by zone (LO 3)

Receptor for angiotensin II on glomerulosa

Direct stimulus by high plasma K⁺ — mechanism (which channel is depolarized?)

ACTH receptor on fasciculata/reticularis (name + G protein)

Why does ACTH only have minor effect on aldosterone?

Why does the glomerulosa NOT make cortisol? (what enzyme is missing?)

Why does the fasciculata make mostly cortisol but not much DHEA?

Zone size, histology, and lifetime changes (LO 3)

Relative SIZE of each cortical zone (% of cortex)

ZF cell histology — special term for lipid-rich foamy cells

ZG cell arrangement (pattern under microscope)

Adrenarche — what event, at what age, in which zone?

DHEA/DHEA-S peak age + trajectory with aging

Premature adrenarche — clinical presentation + age cutoff

Chronic ACTH excess (e.g., Cushing disease) → which zones undergo hyperplasia?

LO 4

ON EXAM — All 3 pathways start with CHOLESTEROL → (StAR + CYP11A1) → PREGNENOLONE. Aldosterone: pregnenolone → progesterone (3β-HSD) → DOC (CYP21) → corticosterone (CYP11B2) → aldosterone (CYP11B2). Cortisol: pregnenolone → 17-OH-pregnenolone (CYP17) → 17-OH-progesterone (3β-HSD) → 11-deoxycortisol (CYP21) → cortisol (CYP11B1). DHEA: 17-OH-pregnenolone → DHEA (CYP17, 17,20-lyase) → androstenedione (3β-HSD) → testosterone (17β-HSD)

Aldosterone pathway (LO 4)

Step	Substrate	Product
1	Cholesterol	Pregnenolone
2	Pregnenolone	Progesterone
3	Progesterone	11-Deoxycorticosterone (DOC)
4	DOC	Corticosterone
5	Corticosterone	Aldosterone

Cortisol pathway (LO 4)

Step	Substrate	Product
1	Cholesterol	Pregnenolone
2	Pregnenolone	17-OH-Pregnenolone
3	17-OH-Pregnenolone	17-OH-Progesterone
4	17-OH-Progesterone	11-Deoxycortisol
5	11-Deoxycortisol	Cortisol

DHEA → androgens pathway (LO 4)

Step	Substrate	Product
1	17-OH-Pregnenolone	DHEA
2	DHEA	DHEA-S (sulfated)
3	DHEA	Androstenedione
4	Androstenedione	Testosterone
5	Testosterone / androstenedione	Estradiol / estrone (peripheral tissues)

The "17-OH shunt": In 21-hydroxylase deficiency (most common CAH), cortisol and aldosterone can't be made → precursor 17-OH-progesterone builds up + substrate is shunted toward androgens → 46,XX VIRILIZATION + salt-wasting crisis.

Key enzymatic checkpoints & clinical ties (LO 4)

Enzyme defect	What's blocked	What's shunted / accumulates	BP / salt effect	Virilization in 46,XX?
21-hydroxylase (CYP21A2) — most common CAH
11β-hydroxylase (CYP11B1)
17α-hydroxylase (CYP17)

Three diagnostic anchors (LO 4)

Screening lab for 21-hydroxylase deficiency

Lab pattern of 11β-hydroxylase deficiency (BP, K⁺, 11-deoxycortisol)

Lab pattern of 17α-hydroxylase (BP, K⁺, sex hormones)

CAH mnemonics — lock these in (LO 4)

HYPER / HYPO rule — which deficiencies cause HTN vs hypotension?

Virilization rule — which deficiencies virilize 46,XX? which does NOT?

Aldosterone pathway mnemonic (enzyme order)

Cortisol pathway mnemonic (enzyme order)

DHEA pathway mnemonic (enzyme order)

CAH clinical spectrum — severity variants (LO 4)

Classic SALT-WASTING CAH — presentation + timing

Simple VIRILIZING CAH (incomplete 21-OH deficiency) — presentation

NON-CLASSIC CAH — presentation (often mistaken for what diagnosis?)

3β-HSD deficiency CAH — expected lab / genital pattern

LO 5

ON EXAM — Aldosterone: loose albumin binding (~60%), very short half-life (~20 min), inactivated by liver → tetrahydroaldosterone-glucuronide. Cortisol: CBG carries ~80% high-affinity + albumin ~15%, half-life ~70-90 min, inactivated by liver (reduction + glucuronidation) AND kidney 11β-HSD2 (cortisol → cortisone). DHEA: weak albumin binding; most circulates as DHEA-S (sulfated in zona reticularis by SULT2A1) — LONGEST half-life of any steroid (8-10 hours), most abundant circulating steroid. Pregnancy ↑ CBG → total cortisol ↑ but free normal. Licorice (glycyrrhetinic acid) inhibits 11β-HSD2 → APPARENT MINERALOCORTICOID EXCESS

Circulation — carrier proteins (LO 5)

Hormone	Primary carrier protein(s) & % bound	Free fraction	Half-life	Clinical pearl
Aldosterone
Cortisol
DHEA (free)
DHEA-sulfate (DHEA-S)

Aldosterone has the SHORTEST half-life (~20 min) of the major adrenal steroids. DHEA-S has the LONGEST (8-10 hr+) and is the MOST ABUNDANT circulating steroid hormone. Cortisol is intermediate at ~70-90 min — long enough to be measured at fixed times of day.

Inactivation — metabolic fate of each hormone (LO 5)

Hormone	Main site of inactivation	Inactivating modification(s)	Final urinary form
Aldosterone
Cortisol
DHEA / DHEA-S

11β-HSD — the cortisol/cortisone switch (LO 5)

11β-HSD2 — where expressed, direction of reaction

Why does the kidney NEED 11β-HSD2 (explain MR specificity paradox)

11β-HSD1 — where expressed, direction of reaction

Licorice (glycyrrhetinic acid) — mechanism of mineralocorticoid-like effect

Congenital 11β-HSD2 deficiency — presentation

Cushing syndrome — why do patients sometimes have HTN + hypokalemia even without aldosterone excess?

Effects of carrier protein changes (LO 5)

Pregnancy / OCPs → CBG?

Liver failure / nephrotic syndrome → CBG?

Why does total cortisol change but FREE cortisol stay the same in these conditions?

What lab should you order instead of total cortisol when CBG is abnormal?

LO 6

ON EXAM — ALDOSTERONE on MR (cytoplasmic) in principal cells of collecting duct: dimer → nucleus → transcribes ENaC (apical), Na-K-ATPase (basolateral), ROMK (apical K⁺ exit), SGK1. Result: Na⁺/H₂O retention + K⁺ and H⁺ secretion. CORTISOL on GR (widely expressed): trans-activation (PEPCK, G6Pase, annexin-1, IκB) + trans-repression (blocks NF-κB, AP-1 → anti-inflammatory). Rapid NON-genomic effects also exist. 11β-HSD2 converts cortisol to cortisone in kidney → protects MR from cortisol (which binds MR as well as aldo does). Spironolactone/eplerenone/finerenone = MR antagonists

Aldosterone at its target: principal cell of the collecting duct (LO 6)

Step	What happens
1. Receptor
2. Resting state of receptor
3. Binding & translocation
4. DNA binding
5. Early gene products (name 3)
6. Apical membrane result
7. Basolateral membrane result
8. Electrolyte / acid-base result

Classic hyperaldosteronism triad: HTN + hypokalemia + metabolic alkalosis. Na⁺/H₂O retention → HTN. K⁺ secretion → hypokalemia. H⁺ secretion by α-intercalated cells → metabolic alkalosis.

Cortisol at its target: glucocorticoid receptor (GR) is widely expressed (LO 6)

Step / effect	Mechanism
Resting GR
Ligand binding
Trans-ACTIVATION (gene ON)
Trans-REPRESSION (gene OFF)
Non-genomic effects (rapid)

Tissue-specific cortisol effects (LO 6)

Liver — metabolic genes induced

Muscle — catabolic effect + mechanism

Adipose — direction of lipolysis + limb vs central fat distribution

Immune cells — main effect

Bone — effect on osteoblasts + osteoclasts

Vasculature — "permissive" effect on what?

MR specificity — the 11β-HSD2 trick (LO 6)

Relative affinity of MR for cortisol vs aldosterone

Relative plasma concentration of cortisol vs aldosterone

Without 11β-HSD2, what would cortisol do at MR?

How does 11β-HSD2 make MR specific for aldosterone?

Two clinical examples of 11β-HSD2 inactivation

Pharmacology of MR & GR (LO 6)

Spironolactone — MOA + 2 non-renal side effects (+ reason)

Eplerenone — MOA + advantage over spironolactone

Finerenone — class + use

Dexamethasone vs hydrocortisone — which has higher GR selectivity / less MR activity?

Fludrocortisone — MR vs GR activity, clinical use

Mifepristone — MOA + endocrine use (in addition to OB use)

Additional clinical pearls & pharmacology (LO 6)

Addisonian crisis — why IV hydrocortisone BEFORE catecholamine pressors?

H⁺-ATPase in α-type intercalated cells — role in hyperaldosteronism lab pattern

Aromatase inhibitors (anastrozole, letrozole, exemestane) — clinical use

Steroidogenesis inhibitors used in Cushing (3 names)

Why does pregnancy / OCPs raise TOTAL cortisol but patient is NOT Cushing?

LO 1–6 · USMLE Step 1 Style

10 questions — weighted across LOs with emphasis on synthesis steps and mechanisms.

Q1. An investigator synthesizes a novel steroid and characterizes its structure as a C21 molecule with a 4-ring nucleus, a ketone at C-3, a double bond between C-4 and C-5, and hydroxyl groups at C-11 (β), C-17 (α), and C-21. The molecule is MOST likely which of the following?

A. Cholesterol
B. Cortisol
C. Testosterone
D. 17β-Estradiol
E. Aldosterone

B — Cortisol.
Cortisol is a C21 pregnane with characteristic hydroxyl groups at C-11β, C-17α, and C-21, plus a 3-oxo-Δ⁴ A-ring (ketone at C-3 with double bond C4-C5). Cholesterol (A) is C27. Testosterone (C) is C19 with only an 11β-OH absent and 17β-OH. Estradiol (D) is C18 with an aromatic A-ring. Aldosterone (E) has a distinctive C-18 aldehyde (hence the name).

Q2. In response to a surge of ACTH, an adrenocortical cell must rapidly deliver cholesterol from its outer mitochondrial membrane to its inner mitochondrial membrane so that steroidogenesis can begin. The protein responsible is:

A. StAR (Steroidogenic Acute Regulatory protein)
B. CYP11A1 (P450scc / desmolase)
C. HMG-CoA reductase
D. LDL receptor
E. Adrenodoxin reductase

A — StAR.
StAR shuttles cholesterol from outer to inner mitochondrial membrane — this is the acute rate-limiting step of steroidogenesis, induced within minutes of ACTH via cAMP/PKA/CREB. CYP11A1 (B) actually CLEAVES cholesterol to pregnenolone on the IMM, but substrate access depends on StAR. Without StAR, no steroids form → congenital lipoid adrenal hyperplasia.

Q3. A 52-year-old woman with a unilateral adrenal adenoma presents with resistant HTN (195/110), serum K⁺ 2.8 mEq/L, and metabolic alkalosis. Plasma aldosterone is markedly elevated; plasma renin is SUPPRESSED. The excess hormone is made in which adrenal zone and by which signature enzyme?

A. Zona glomerulosa; aldosterone synthase (CYP11B2)
B. Zona fasciculata; 11β-hydroxylase (CYP11B1)
C. Zona reticularis; 17,20-lyase (CYP17)
D. Adrenal medulla; phenylethanolamine N-methyltransferase
E. Zona fasciculata; 21-hydroxylase (CYP21A2)

A — Zona glomerulosa; aldosterone synthase.
Conn syndrome (primary hyperaldosteronism) from an adrenal adenoma. Aldosterone synthase (CYP11B2) is uniquely expressed in zona glomerulosa — it uses DOC and corticosterone as substrates and performs the final hydroxylation and C18 oxidation to yield aldosterone. CYP11B1 (B) in fasciculata makes cortisol (different terminal enzyme). The clue “low renin” = PRIMARY (autonomous aldosterone suppresses RAAS).

Q4. A 2-week-old 46,XX infant presents with ambiguous genitalia, dehydration, vomiting, hyponatremia, hyperkalemia, and elevated serum 17-OH-progesterone. Which enzyme is deficient and which precursor is shunted to cause virilization?

A. 11β-hydroxylase; shunt to 11-deoxycorticosterone
B. 21-hydroxylase; shunt from 17-OH-progesterone into the androgen pathway (DHEA, androstenedione)
C. 17α-hydroxylase; shunt to aldosterone
D. 3β-HSD; shunt to mineralocorticoid pathway
E. Aldosterone synthase; shunt to cortisol

B — 21-hydroxylase deficiency (classic salt-wasting CAH).
The MOST common CAH. 21-hydroxylase converts 17-OH-progesterone → 11-deoxycortisol (cortisol path) and progesterone → DOC (aldo path). When blocked: cortisol ↓, aldosterone ↓ (→ hyponatremia, hyperkalemia, salt-wasting crisis), and 17-OH-progesterone accumulates → shunted via 17,20-lyase toward DHEA/androstenedione → VIRILIZATION of 46,XX infants. Screening lab: elevated 17-OH-progesterone.

Q5. Which of the following steroid hormones has the LONGEST plasma half-life and is the MOST abundant circulating adrenal steroid?

A. Aldosterone
B. Cortisol
C. Free DHEA
D. DHEA-sulfate (DHEA-S)
E. 17-OH-progesterone

D — DHEA-sulfate.
DHEA-S is made in the zona reticularis by SULT2A1 (sulfotransferase). Sulfation dramatically prolongs half-life (8–10+ hours) by slowing clearance, making DHEA-S the most abundant circulating steroid hormone in humans. Aldosterone (A) has the shortest half-life (~20 min, loose albumin binding only). Cortisol is ~70–90 min. Free DHEA is short (~30 min); sulfation is the trick that extends it.

Q6. In principal cells of the cortical collecting duct, aldosterone binding to the mineralocorticoid receptor increases transcription of a Na⁺ channel in the apical membrane. The gene product is:

A. SGLT2 (sodium-glucose cotransporter 2)
B. ENaC (epithelial Na⁺ channel)
C. NKCC2 (Na-K-2Cl cotransporter)
D. NCC (Na-Cl cotransporter)
E. ROMK (renal outer medullary K⁺)

B — ENaC.
Aldosterone binds cytoplasmic MR → dimer → nucleus → transcription of ENaC (apical Na⁺ channel), Na-K-ATPase (basolateral), ROMK (apical K⁺ exit — answer E is in the cell but is distinct), and SGK1. Amiloride blocks ENaC; spironolactone blocks MR upstream. NKCC2 is in thick ascending limb; NCC is in distal convoluted tubule; SGLT2 is in proximal tubule.

Q7. The mineralocorticoid receptor (MR) binds cortisol and aldosterone with SIMILAR affinity. Because circulating cortisol concentrations are ~100–1000× higher than aldosterone, why doesn't cortisol chronically activate MR in the kidney?

A. Cortisol cannot cross the plasma membrane of collecting duct cells
B. 11β-HSD2 in collecting duct cells converts cortisol to cortisone, which does NOT bind MR
C. Cortisol is selectively cleared by the liver before reaching the kidney
D. MR is expressed only on apical membranes, where aldosterone uniquely acts
E. CBG prevents cortisol from entering cells

B — 11β-HSD2.
Kidney collecting duct cells express 11β-HSD2, which oxidizes cortisol's 11β-OH to an 11-ketone, producing cortisone. Cortisone has low affinity for MR, so cortisol doesn't activate MR. Licorice (glycyrrhetinic acid) inhibits 11β-HSD2 → cortisol activates MR → “apparent mineralocorticoid excess” (HTN + hypokalemia + low aldo + low renin). Congenital 11β-HSD2 deficiency = same phenotype.

Q8. A 28-year-old man with HIV/AIDS is evaluated for persistent fatigue. Morning cortisol is low-normal; ACTH is markedly elevated; skin hyperpigmentation is noted on his buccal mucosa and palmar creases. Which BEST explains the hyperpigmentation?

A. Excess aldosterone stimulates melanocytes
B. Low cortisol releases pituitary POMC transcription → ACTH and α-MSH rise together (MSH is a POMC cleavage product that activates melanocortin receptors)
C. Accumulated 17-OH-progesterone is a melanocyte stimulant
D. Dexamethasone suppresses melanogenesis
E. 11β-HSD2 deficiency causes pigment retention

B — POMC cleavage produces both ACTH and α-MSH.
Primary adrenal insufficiency (Addison disease, here from HIV → adrenal infection/destruction) → low cortisol → loss of negative feedback → pituitary POMC transcription soars. POMC is cleaved into ACTH + β-endorphin + α-MSH. Elevated α-MSH (and ACTH itself, which also has some melanocortin activity) activates MC1R on melanocytes → hyperpigmentation (palmar creases, buccal mucosa, scars). Secondary adrenal insufficiency (pituitary cause) does NOT cause hyperpigmentation because ACTH/POMC are low.

Q9. A 45-year-old woman has Cushing syndrome. Her physician considers two MR antagonists: spironolactone and eplerenone. Which is TRUE of eplerenone relative to spironolactone?

A. Eplerenone has HIGHER androgen-receptor activity and is more likely to cause gynecomastia
B. Eplerenone is MORE selective for MR, with less progesterone-receptor and androgen-receptor activity → fewer endocrine side effects (less gynecomastia, less menstrual irregularity)
C. Eplerenone blocks only ENaC and spares MR
D. Eplerenone is a potent GR antagonist
E. Eplerenone has no effect on K⁺ balance

B — Eplerenone is more MR-selective than spironolactone.
Spironolactone is a promiscuous steroid-receptor antagonist — MR block + anti-androgen effects + progesterone-receptor effects → gynecomastia (men), menstrual irregularities (women), decreased libido. Eplerenone is more MR-selective → fewer sex-hormone side effects but still K⁺-sparing. Both used in hyperaldosteronism, heart failure, and resistant HTN. Finerenone is a newer nonsteroidal MR antagonist used in diabetic CKD.

Q10. Which of the following is the FIRST committed step of steroid biosynthesis (shared by all steroidogenic tissues), catalyzed in the INNER mitochondrial membrane?

A. Cholesterol transport into mitochondria by StAR
B. Conversion of cholesterol to pregnenolone by CYP11A1 (side-chain cleavage)
C. Hydroxylation of pregnenolone at C-17 by CYP17
D. 21-hydroxylation of 17-OH-progesterone by CYP21A2
E. 11β-hydroxylation of 11-deoxycortisol by CYP11B1

B — CYP11A1 (cholesterol → pregnenolone).
CYP11A1 (= P450scc = desmolase) catalyzes cleavage of the cholesterol side chain on the INNER mitochondrial membrane, producing pregnenolone. This is the first committed step of steroidogenesis — the first enzymatic reaction irreversibly dedicated to the steroid pathway. StAR (A) is the RATE-LIMITING shuttle (transport, not enzymatic conversion). CYP17 (C), CYP21 (D), CYP11B1 (E) are downstream, pathway-specific enzymes.

Adrenal Hormone Synthesis & Action — Active Recall