CAH enzyme defects: 21 vs 11β vs 17α hydroxylase

Start with androgen direction, then mineralocorticoid effect. 21-hydroxylase deficiency causes androgen excess and may cause salt wasting; 11β-hydroxylase deficiency causes androgen excess with deoxycorticosterone-driven low-renin hypertension; 17α-hydroxylase/17,20-lyase deficiency causes low sex steroids with low-renin hypertension. Use the characteristic accumulated steroids to confirm the pattern. These are exam distinctions, not substitutes for specialist biochemical diagnosis.

Clinical review required - completed: Jack Newland independently reviewed the final article, comparison table and cited sources on 15 November 2026. This is an editorial accuracy review and does not claim specialist endorsement or professional credentials.

This article is for professional MRCPCH examination education. It does not provide patient-specific diagnosis, treatment, prescribing, dosing or emergency-management instructions.

The direct answer: first decide whether androgens are high or low

The three enzyme defects become easier when you stop trying to memorise the whole steroid pathway at once. Use two questions:

1. Are androgens increased or decreased?
2. Is the mineralocorticoid pattern salt wasting or low-renin hypertension?

That produces a compact exam grid:

• 21-hydroxylase deficiency: androgen up; mineralocorticoid effect down in the salt-wasting form.
• 11β-hydroxylase deficiency: androgen up; mineralocorticoid effect up because deoxycorticosterone accumulates.
• 17α-hydroxylase/17,20-lyase deficiency: androgen down; mineralocorticoid effect up because deoxycorticosterone and corticosterone pathways remain available.

The memorable pattern is therefore 21: salt down, sex up; 11: pressure up, sex up; 17: pressure up, sex down. "Sex" here is only a shorthand for androgen or sex-steroid production. It is not a description of sex, gender identity or every individual phenotype.

Key points

• Both 21-hydroxylase and 11β-hydroxylase deficiencies reduce cortisol and divert precursors towards androgen production. The best discriminator is mineralocorticoid effect: salt wasting can occur in classic 21-hydroxylase deficiency, whereas deoxycorticosterone excess produces a low-renin hypertensive pattern in 11β-hydroxylase deficiency.
• 17α-hydroxylase/17,20-lyase deficiency also produces a low-renin hypertensive pattern, but androgen and oestrogen synthesis are reduced rather than increased. Pubertal failure or undervirilisation therefore points away from the two androgen-excess defects.
• Exam mnemonics describe typical directions, not universal bedside findings. Blood pressure can be normal early in 11β-hydroxylase deficiency, hypokalaemia is not invariable, and 21-hydroxylase deficiency ranges from salt-wasting to simple-virilising and non-classic phenotypes.

Why all three defects reduce cortisol

The adrenal cortex produces mineralocorticoids, glucocorticoids and adrenal androgens through connected pathways. Each enzyme converts one or more precursors into downstream products. Blocking an enzyme reduces products beyond the block and increases some upstream precursors.

All three disorders in this comparison impair cortisol synthesis. Reduced cortisol feedback increases adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex and helps explain the "hyperplasia" in congenital adrenal hyperplasia (CAH). The distinguishing pattern comes from where the extra precursor flow can still go.

The current MRCPCH theory syllabus explicitly includes adrenal anatomy and function, the pathophysiology and investigation of adrenal disorders, endocrine hormone regulation, and the presentation of adrenal disease. This makes the comparison particularly relevant to the TAS component described in the MRCPCH exam guide, while recognition and interpretation can also appear in FOP- and AKP-style reasoning.

Compare the three enzyme defects in one table

Feature	21-hydroxylase deficiency	11β-hydroxylase deficiency	17α-hydroxylase/17,20-lyase deficiency
Gene	CYP21A2	CYP11B1	CYP17A1
Inheritance	Autosomal recessive	Autosomal recessive	Autosomal recessive
Cortisol	Reduced	Reduced	Reduced, with corticosterone providing some glucocorticoid activity
Androgen direction	Increased	Increased	Decreased
Mineralocorticoid pattern	Deficient in classic salt-wasting disease; preserved enough to avoid overt salt wasting in simple-virilising and non-classic forms	Deoxycorticosterone excess causes low-renin hypertension; aldosterone is often suppressed	Deoxycorticosterone and corticosterone excess causes low-renin hypertension; aldosterone is suppressed
Typical blood pressure clue	Low or normal in salt-wasting presentation; hypotension may occur with adrenal crisis	Hypertension is an important clue but may not be present early	Hypertension is a major clue and may present in childhood or adolescence
Typical potassium clue	Hyperkalaemia in salt-wasting disease	Hypokalaemia may occur but is not universal	Hypokalaemia is common but not universal
Characteristic biochemical clue	Markedly raised 17-hydroxyprogesterone in classic disease	Raised 11-deoxycortisol and deoxycorticosterone	Raised deoxycorticosterone, corticosterone and progesterone with low sex steroids
Reproductive-development clue	Prenatal androgen excess can virilise external genitalia in affected 46,XX fetuses; 46,XY external genitalia are usually typical at birth	Similar androgen-excess pattern to 21-hydroxylase deficiency	Low sex steroids can cause undervirilisation in 46,XY individuals and absent or incomplete pubertal development in both 46,XY and 46,XX individuals
Fast discriminator	Androgens up + possible salt loss	Androgens up + pressure up	Androgens down + pressure up

This table deliberately uses directions rather than absolute values. Assay methods, age, phenotype severity and treatment status affect measured concentrations. A complete steroid profile and specialist interpretation are required in real clinical practice.

21-hydroxylase deficiency: androgen excess with possible salt wasting

21-hydroxylase normally participates in two important conversions: progesterone towards deoxycorticosterone in the mineralocorticoid pathway, and 17-hydroxyprogesterone towards 11-deoxycortisol in the cortisol pathway. When the enzyme is deficient, 17-hydroxyprogesterone and other precursors accumulate and are diverted towards androgen synthesis.

That explains the core exam pattern:

• cortisol falls;
• androgens rise;
• 17-hydroxyprogesterone rises; and
• aldosterone synthesis may be inadequate in severe classic disease.

Classic 21-hydroxylase deficiency is usefully divided into salt-wasting and simple-virilising phenotypes, while non-classic disease is milder. These categories are exam-friendly, but the 2026 Endotext chapter emphasises that severity is a continuum.

In the salt-wasting form, the combination of inadequate mineralocorticoid effect and cortisol deficiency can produce hyponatraemia, hyperkalaemia, dehydration and hypotension. In simple-virilising disease, enough aldosterone production remains to prevent overt salt wasting. Non-classic disease does not present with neonatal salt wasting or prenatal virilisation.

The 21-hydroxylase trap

Do not turn "CAH equals salt wasting" into a universal rule. Salt wasting is characteristic of severe classic 21-hydroxylase deficiency, not every CAH phenotype. Conversely, normal-appearing external genitalia in a 46,XY newborn does not exclude salt-wasting 21-hydroxylase deficiency.

17-hydroxyprogesterone is the central screening and diagnostic marker for 21-hydroxylase deficiency, but it is not a stand-alone label for every context. The Endocrine Society guideline recommends a complete adrenocortical profile when values are borderline or another enzyme defect must be distinguished.

11β-hydroxylase deficiency: androgen excess with hypertension

11β-hydroxylase converts 11-deoxycortisol to cortisol and deoxycorticosterone to corticosterone. The block therefore reduces cortisol but allows 11-deoxycortisol and deoxycorticosterone to accumulate.

Upstream steroid flow is diverted towards androgen synthesis, so the androgen phenotype resembles 21-hydroxylase deficiency. The decisive difference is deoxycorticosterone (DOC). DOC has mineralocorticoid activity and can cause sodium retention, suppressed renin and hypertension even though aldosterone itself is low.

The exam pattern is:

• cortisol down;
• androgens up;
• 11-deoxycortisol and DOC up;
• renin down; and
• blood pressure often up.

The current Endotext review reports that hypertension develops in approximately two-thirds of affected people over time, but it may be absent in infancy and childhood. Hypokalaemia is relatively uncommon in children and does not reliably track with blood pressure. A stem does not need to include every classic feature.

The 11β-hydroxylase trap

Do not reason that low aldosterone must mean salt wasting. In 11β-hydroxylase deficiency, aldosterone is suppressed because DOC is already providing mineralocorticoid activity and suppressing renin. The functional mineralocorticoid effect - not the aldosterone concentration alone - explains the hypertension.

Another trap is confusing 11-deoxycortisol with cortisol. The prefix matters: the precursor immediately before the blocked 11β step rises, while its cortisol product falls.

17α-hydroxylase deficiency: hypertension with low sex steroids

CYP17A1 has both 17α-hydroxylase and 17,20-lyase activities in the adrenal glands and gonads. Most affected people have impairment of both activities. Cortisol synthesis falls, but steroid flow continues through the 17-deoxy pathway towards deoxycorticosterone and corticosterone.

This creates the apparently paradoxical combination of reduced cortisol with relatively mild glucocorticoid-deficiency symptoms: corticosterone has glucocorticoid activity. At the same time, excess DOC produces sodium retention, suppressed renin, hypertension and sometimes hypokalaemia.

Unlike 21- and 11β-hydroxylase deficiencies, the pathway to sex steroids is also impaired. The central pattern is therefore:

• cortisol down;
• androgens and oestrogens down;
• DOC and corticosterone up;
• renin down; and
• blood pressure up.

In complete deficiency, 46,XY individuals may have undervirilised external genitalia, while 46,XX individuals usually have typical external genitalia at birth but fail to progress through normal puberty without appropriate specialist care. Sparse pubic or body hair, absent breast development and primary amenorrhoea may appear in an adolescent stem. Partial defects create more variable phenotypes.

The 17α-hydroxylase trap

Candidates often group 17α-hydroxylase deficiency with 11β-hydroxylase deficiency because both can cause low-renin hypertension and hypokalaemia. The androgen direction separates them:

• 11β block: androgens rise;
• 17α block: sex steroids fall.

Do not say that low cortisol always produces adrenal crisis. In 17α-hydroxylase/17,20-lyase deficiency, increased corticosterone generally supplies enough glucocorticoid activity to prevent the typical adrenal-insufficiency presentation, even though measured cortisol is low.

How an MRCPCH-style stem may test the distinction

These examples are original educational patterns, not recalled examination questions.

Pattern 1: salt loss plus androgen excess

A young infant has poor weight gain, dehydration, hyponatraemia and hyperkalaemia. The stem also signals prenatal androgen excess and a markedly raised 17-hydroxyprogesterone. The paired clues are salt wasting plus androgen excess, pointing towards classic salt-wasting 21-hydroxylase deficiency.

Pattern 2: androgen excess plus hypertension

A child has early androgen effects, advanced skeletal maturation and hypertension. Renin is suppressed, while 11-deoxycortisol and DOC are raised. The paired clues are androgen excess plus mineralocorticoid excess, pointing towards 11β-hydroxylase deficiency.

Pattern 3: pubertal failure plus hypertension

An adolescent has absent or incomplete pubertal development, sparse body hair, hypertension and hypokalaemia. Sex steroids and renin are low, while DOC and corticosterone are raised. The paired clues are sex-steroid deficiency plus mineralocorticoid excess, pointing towards 17α-hydroxylase/17,20-lyase deficiency.

The MRCPCH exam-format guide explains how these concepts map across FOP, TAS and AKP. For question practice, retrieve the two-axis pattern before looking at the options; recognition after reading the answer is weaker evidence than producing the distinction independently.

Common mistakes and how to correct them

Memorising enzyme numbers without pathway consequences

Replace isolated numbers with a causal chain:

block → accumulated precursor → available escape pathway → androgen direction → mineralocorticoid effect

A concept map can show this more faithfully than a long acronym because the arrows explain why the phenotype occurs.

Treating every clue as absolute

"11 causes hypertension" is useful but incomplete. Blood pressure may be normal early. "11 causes hypokalaemia" is also unreliable as an absolute. Use the complete cluster and the characteristic steroid marker.

Confusing aldosterone concentration with mineralocorticoid effect

Both 11β- and 17α-hydroxylase deficiencies can have suppressed aldosterone while still producing a mineralocorticoid-excess phenotype. DOC is the missing explanatory variable.

Forgetting phenotype severity

Classic salt-wasting, simple-virilising and non-classic 21-hydroxylase deficiency are not interchangeable. Partial 17α-hydroxylase defects also create variable reproductive and biochemical findings. Exam tables provide anchors; real phenotypes remain continuous.

Using a mnemonic without decoding it

The MRCPCH mnemonic guide recommends testing whether a cue reproduces the underlying explanation. For this topic, always decode "pressure up" into DOC accumulation and renin suppression.

A five-step recall check

For each enzyme, retrieve these five fields without looking:

1. blocked conversion or characteristic accumulated precursor;
2. androgen direction;
3. mineralocorticoid effect;
4. blood pressure and potassium tendency; and
5. the most discriminating presentation clue.

Then change the starting cue. Begin once with the enzyme name, once with the electrolyte pattern and once with the reproductive-development clue. This prevents the table order from becoming the only route to the answer. The format-matching guide explains why varied retrieval should eventually be checked in fresh single-best-answer questions.

Conclusion

The cleanest way to distinguish these CAH enzyme defects is to use androgen direction and mineralocorticoid effect before reaching for a mnemonic.

Remember the two-axis grid: 21 is salt down, sex up; 11 is pressure up, sex up; 17 is pressure up, sex down. Then confirm the answer with the accumulated steroid: 17-hydroxyprogesterone for 21-hydroxylase deficiency, 11-deoxycortisol and DOC for 11β-hydroxylase deficiency, and DOC/corticosterone with low sex steroids for 17α-hydroxylase/17,20-lyase deficiency.

Frequently asked questions

Which CAH enzyme deficiency is most common?

21-hydroxylase deficiency is by far the most common form of CAH. The Endocrine Society guideline attributes approximately 95% of CAH to CYP21A2 variants. 11β-hydroxylase deficiency is the next most common enzyme defect, while 17α-hydroxylase/17,20-lyase deficiency is rare.

Which CAH enzyme deficiencies cause hypertension?

11β-hydroxylase and 17α-hydroxylase/17,20-lyase deficiencies classically cause low-renin hypertension because deoxycorticosterone accumulates. Hypertension may not be evident early in 11β-hydroxylase deficiency, and neither hypertension nor hypokalaemia should be treated as an invariable finding.

Which CAH enzyme deficiency causes salt wasting?

Severe classic 21-hydroxylase deficiency commonly causes salt wasting because aldosterone synthesis is inadequate. Simple-virilising and non-classic 21-hydroxylase deficiency do not share the same neonatal salt-wasting pattern. Other rarer CAH disorders can also cause salt wasting, so the feature is not unique to 21-hydroxylase deficiency across the entire CAH differential.

What is the best clue for 11β-hydroxylase deficiency?

The high-yield combination is androgen excess with a low-renin hypertensive pattern. Raised 11-deoxycortisol and deoxycorticosterone support the enzyme-level distinction from 21-hydroxylase deficiency. Blood pressure alone is insufficient because hypertension can be absent early.

How do I separate 11β- from 17α-hydroxylase deficiency?

Both can produce deoxycorticosterone-driven hypertension and sometimes hypokalaemia. Separate them by sex-steroid direction: androgens are increased in 11β-hydroxylase deficiency but decreased in 17α-hydroxylase/17,20-lyase deficiency. Pubertal failure or undervirilisation therefore favours the 17α defect.

Sources

1. Royal College of Paediatrics and Child Health. Theory exams: structure and syllabi. Accessed 15 November 2026.
2. Royal College of Paediatrics and Child Health. MRCPCH Theory examination syllabi, version 2. Endocrinology and Growth, pp. 23 - 24.
3. Speiser PW, White PC. Congenital Adrenal Hyperplasia. Endotext. Updated 21 January 2026.
4. Speiser PW, Arlt W, Auchus RJ, et al. Congenital adrenal hyperplasia due to steroid 21-hydroxylase deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism. 2018;103(11):4043 - 4088.
5. İsakoca M, Erdeve Ş, Çetinkaya S. Rare types of congenital adrenal hyperplasias other than 21-hydroxylase deficiency. Journal of Clinical Research in Pediatric Endocrinology. 2025;17(Suppl 1):23 - 32.
6. Khattab A, Haider S, Kumar A, et al. Clinical, genetic and structural basis of congenital adrenal hyperplasia due to 11β-hydroxylase deficiency. Proceedings of the National Academy of Sciences. 2017;114(10):E1933 - E1940.
7. Auchus RJ. Steroid 17-hydroxylase and 17,20-lyase deficiencies, genetic and pharmacologic. Journal of Steroid Biochemistry and Molecular Biology. 2017;165(Pt A):71 - 78.
8. PaedVault. Acceptable Use Policy. Updated 28 July 2026.