Everything You Need to Know About Congenital adrenal hyperplasia (21-hydroxylase, 11-hydroxylase, 17-hydroxylase) for Step 1

Congenital adrenal hyperplasia (CAH) is one of those Step 1 topics that looks like pure memorization—until you realize it’s really just pattern recognition built off one steroid pathway. If you can predict what builds up upstream of a blocked enzyme and what hormones fall downstream, you can answer almost any CAH question (including the tricky blood pressure and sex development stems).

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Big Picture: What Is CAH?

Congenital adrenal hyperplasia (CAH) = a group of autosomal recessive enzyme deficiencies in adrenal steroid synthesis that cause:

• ↓ Cortisol (often) → ↑ ACTH (loss of negative feedback) → bilateral adrenal hyperplasia
• Shunting of precursors into other pathways → excess androgens or excess mineralocorticoid activity, depending on the enzyme

The Steroid Pathway “3 Outputs”

Think adrenal cortex products as 3 major endpoints:

Mnemonic: GFR = Salt, Sugar, Sex (outer → inner).

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The Enzymes You Must Know (and how Step 1 tests them)

The classic Step framing:

1. 21-hydroxylase deficiency (most common)
2. 11β-hydroxylase deficiency
3. 17α-hydroxylase deficiency

A high-yield way to organize: blood pressure + potassium + androgens + renin.

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21-Hydroxylase Deficiency (Most Common)

Pathophysiology

21-hydroxylase is needed for:

• Progesterone → 11-deoxycorticosterone (DOC) (mineralocorticoid path)
• 17-hydroxyprogesterone → 11-deoxycortisol (glucocorticoid path)

So deficiency causes:

• ↓ Cortisol
• ↓ Aldosterone
• ↑ ACTH → adrenal hyperplasia
• ↑ Androgens (shunting toward sex steroids)
• ↑ 17-hydroxyprogesterone (classic diagnostic clue)

Clinical Presentation

Two classic variants tested:

1) Salt-wasting (classic, severe)

• Hypotension
• Hyponatremia
• Hyperkalemia
• Dehydration, vomiting, shock (often in first 1–3 weeks of life)
• ↑ Renin

2) Simple virilizing (less severe)

• Enough mineralocorticoid to avoid shock, but still ↑ androgens

Sex development findings:

• 46,XX: ambiguous genitalia / virilization at birth (clitoromegaly, labioscrotal fusion)
• 46,XY: normal male external genitalia at birth; later precocious puberty / rapid growth, early epiphyseal closure

Diagnosis (Step-style)

• Newborn screen: ↑ 17-hydroxyprogesterone
• Labs (classic salt-wasting):
  • ↓ Cortisol
  • ↓ Aldosterone
  • ↑ ACTH
  • ↑ Androgens (DHEA, androstenedione)
  • ↑ Renin
  • Metabolic acidosis may be present due to volume depletion

Treatment

• Glucocorticoid replacement (e.g., hydrocortisone in children) to replace cortisol and suppress ACTH
• Mineralocorticoid replacement (e.g., fludrocortisone) + salt supplementation in salt-wasting forms
• Acute adrenal crisis: IV hydrocortisone + isotonic fluids, correct electrolytes

HY Associations

• Most common cause of ambiguous genitalia in newborn girls (46,XX)
• Step loves: “↓ cortisol, ↓ aldosterone, ↑ androgens”
• ↑ 17-hydroxyprogesterone is the classic “name that CAH” lab

First Aid cross-reference: Endocrine → Adrenal cortex → Congenital adrenal hyperplasia (21-hydroxylase deficiency)

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11β-Hydroxylase Deficiency

Pathophysiology

11β-hydroxylase converts:

• 11-deoxycortisol → cortisol
• DOC → corticosterone (leading toward aldosterone)

Deficiency causes:

• ↓ Cortisol → ↑ ACTH
• ↑ Androgens (shunting)
• ↑ DOC (key!) which has mineralocorticoid activity
  • Leads to HTN and hypokalemia
• ↓ Renin (suppressed by hypertension/volume expansion)
• Aldosterone is often low (suppressed), but DOC “acts like aldosterone”

Clinical Presentation

• Hypertension (often the giveaway versus 21-hydroxylase)
• Hypokalemia, metabolic alkalosis (mineralocorticoid effect)
• Virilization:
  • 46,XX: ambiguous genitalia
  • 46,XY: precocious puberty

Diagnosis

• ↑ 11-deoxycortisol (classic lab clue)
• ↑ DOC
• ↓ Renin
• ↓ Cortisol, ↑ ACTH, ↑ androgens

Treatment

• Glucocorticoids to suppress ACTH (reduces androgen + DOC overproduction)
• Manage hypertension if needed (but often improves with steroid replacement)

HY Associations

• Think: “11 deficiency = 1 (DOC) causes 1 problem: hypertension”
• Distinguish from 21-hydroxylase: both virilize, but only 11β causes HTN

First Aid cross-reference: Endocrine → Adrenal cortex → Congenital adrenal hyperplasia (11β-hydroxylase deficiency)

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17α-Hydroxylase Deficiency

Pathophysiology

17α-hydroxylase is necessary for making:

• Cortisol (glucocorticoids)
• Sex steroids (androgens/estrogens)

So deficiency causes:

• ↓ Cortisol → ↑ ACTH
• ↓ Androgens and ↓ estrogens
• Precursors are shunted into mineralocorticoid pathway:
  • ↑ DOC and ↑ corticosterone → hypertension and hypokalemia
• ↓ Renin
• Aldosterone is typically low (suppressed), despite mineralocorticoid effects being high from DOC

Clinical Presentation (very testable)

1) Hypertension + hypokalemia (mineralocorticoid effect)

2) Sexual development problems due to low sex steroids

• 46,XY: undervirilized male → ambiguous/female external genitalia, undescended testes, absent internal female organs (MIS still produced by testes)
• 46,XX: normal internal organs, but lack of secondary sexual characteristics at puberty:
  • primary amenorrhea
  • absent breast development (low estrogen)
  • decreased pubic/axillary hair (low androgens)

Diagnosis

• ↓ Cortisol
• ↓ Androgens (DHEA)
• ↑ DOC (mineralocorticoid activity) → ↓ renin
• ↑ ACTH
• Sometimes ↑ corticosterone (can partially substitute for cortisol effects, so adrenal crisis may be less dramatic than 21-hydroxylase)

Treatment

• Glucocorticoids (suppress ACTH → lower DOC)
• Sex hormone replacement as needed (e.g., estrogen ± progesterone in 46,XX; testosterone in 46,XY depending on goals)
• Manage hypertension if persistent

HY Associations

• Think: “17 deficiency = no sex hormones → sexual infantilism; plus HTN”
• The CAH that causes decreased androgens (opposite of 21 and 11)

First Aid cross-reference: Endocrine → Adrenal cortex → Congenital adrenal hyperplasia (17α-hydroxylase deficiency)

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The Money Table: Compare the 3 Classic CAH Types

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How CAH Shows Up in Question Stems (Pattern Recognition)

Pattern 1: Ambiguous genitalia + shock in a newborn

• Think 21-hydroxylase
• Confirm with ↑ 17-hydroxyprogesterone
• Treat emergently as adrenal crisis

Pattern 2: Virilization/precocious puberty + hypertension

• Think 11β-hydroxylase
• Mechanism: DOC excess

Pattern 3: Hypertension + hypokalemia + absent puberty / undervirilized male

• Think 17α-hydroxylase
• Mechanism: DOC excess + low sex steroids

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Diagnosis Workflow (Practical Step 1/2 approach)

1. Assess hemodynamics and electrolytes
   • Hypotension + hyperK → 21
   • Hypertension + hypoK → 11 or 17
2. Assess androgens/sexual development
   • High androgens (virilization) → 21 or 11
   • Low androgens (sexual infantilism) → 17
3. Use the “signature metabolite”
   • ↑ 17-hydroxyprogesterone → 21
   • ↑ 11-deoxycortisol → 11
   • Low androgens + high DOC phenotype → 17

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Treatment Principles (Why Steroids Fix Multiple Problems)

Core concept: Most CAH problems start with ↓ cortisol → ↑ ACTH → adrenal hyperplasia + precursor overflow.

So giving glucocorticoids:

• replaces cortisol
• suppresses ACTH
• reduces overproduction of androgens (21, 11) and DOC (11, 17)

Add mineralocorticoids only when true deficiency exists (classically 21-hydroxylase salt-wasting).

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High-Yield One-Liners to Memorize

• 21-hydroxylase deficiency: “Salt-wasting + virilization; ↑ 17-OHP.”
• 11β-hydroxylase deficiency: “Virilization + hypertension (DOC).”
• 17α-hydroxylase deficiency: “Hypertension + hypogonadism (low sex steroids).”
• All are autosomal recessive and cause ↑ ACTH → adrenal hyperplasia (unless cortisol is adequately replaced).

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Quick Self-Quiz (Step-style)

1. Newborn with vomiting, weight loss, dehydration, hyperkalemia, ambiguous genitalia.
   → 21-hydroxylase deficiency, test 17-hydroxyprogesterone, treat with hydrocortisone + fludrocortisone.

2. Child with early puberty signs, acne, virilization, hypertension, hypokalemia.
   → 11β-hydroxylase deficiency, due to ↑ DOC.

3. Teen girl with primary amenorrhea, no breast development, hypertension, low K⁺, low androgens.
   → 17α-hydroxylase deficiency.