Classic 21-OHD CAH results from biallelic pathogenic variants in CYP21A2, located on chromosome 6, and is inherited in an autosomal recessive pattern. Individuals must inherit two pathogenic variants, one from each parent, to be affected; when both parents are carriers, each pregnancy carries a 25% probability of producing an affected child.

The salt-wasting form typically arises from severe pathogenic variants on both CYP21A2 alleles, with near-complete or complete loss of 21-hydroxylase enzyme activity. Genotype-phenotype correlations are imperfect; individuals with the same genotype can exhibit differing phenotypic severity, particularly in the simple virilizing form where phenotypic variability is most pronounced. Approximately 1% of pathogenic variants arise de novo, with only one parent being a carrier. CYP21A2 resides in a chromosomal region containing a highly homologous pseudogene, making gene deletions and gene conversion events common pathogenic mechanisms.

Classic 21-OHD CAH is suspected in females presenting with external genital virilization at birth, infants of either sex presenting with a salt-wasting crisis in the first four weeks of life, and infants with an elevated 17-OHP concentration detected on newborn screening. Affected individuals have a normal karyotype (46,XX in females and 46,XY in males).

Newborn screening for 21-OHD CAH identifies infants at risk for life-threatening salt-wasting crises, particularly males who lack the external diagnostic indicator of ambiguous genitalia. HHS recommends this condition for inclusion in state newborn-screening panels; actual state panels vary. Dried blood spot 17-OHP measurement is the primary screening method; liquid chromatography-tandem mass spectrometry serves as a second-tier confirmatory test in many programs to reduce false positives in preterm or low-birth-weight infants.

Diagnostic confirmation involves markedly elevated serum 17-OHP and adrenal androgens, with electrolyte abnormalities consistent with salt wasting in the salt-wasting subtype, and identification of biallelic pathogenic variants in CYP21A2. The ACTH stimulation test characterizes the degree of 21-hydroxylase enzyme deficiency and helps distinguish the classic from the non-classic form. Vaginography may be used to assess anatomical details in affected females, and bone age assessment documents the degree of skeletal maturation from androgen exposure.

Management of classic 21-OHD CAH is carried out by a multidisciplinary team typically including specialists in pediatric endocrinology, pediatric urology or surgery, clinical genetics, and psychology. Treatment objectives include replacing deficient steroids, suppressing ACTH-driven adrenal androgen overproduction, preventing virilization, supporting normal growth, and preserving fertility potential.

Glucocorticoid replacement is the foundational therapy for all individuals with the classic form, addressing cortisol deficiency and reducing adrenal androgen excess. Hydrocortisone in tablet form is the standard glucocorticoid preparation during growth in children; more potent glucocorticoids are used in adults after linear growth is complete. Overtreatment with glucocorticoids produces iatrogenic hypercortisolism and is monitored through serial hormonal and growth assessments.

Mineralocorticoid replacement is a necessary component of management for individuals with the salt-wasting form and is initiated in the newborn period; sodium chloride supplementation may be added in early infancy, with requirements adjusted over time.

Physiologic stress—including febrile illness, gastroenteritis with dehydration, surgery under general anesthesia, and major trauma—increases glucocorticoid requirements in individuals with classic 21-OHD CAH, with adrenal crisis a risk if replacement is not adjusted accordingly. Emergency glucocorticoid protocols are a standard component of management planning for affected individuals.

Genitoplasty is an option for females with severe virilization (Prader III-V), undertaken within a multidisciplinary framework with emphasis on functional outcome and preservation of clitoral innervation.

Long-term management includes structured surveillance. During active childhood growth, serial measurements of hormonal markers—including 17-OHP, 4-androstenedione, and testosterone—along with linear growth velocity, bone age, and pubertal progression are obtained at regular intervals. Blood pressure and plasma renin activity or direct renin assay track mineralocorticoid replacement adequacy. In males, periodic testicular imaging for adrenal rest tissue begins after puberty. In adults, surveillance encompasses fertility assessment, lipid profile, bone mineral density, and blood pressure monitoring. Patient assistance programs may be available to help cover treatment costs.

The prognosis for classic 21-OHD CAH depends on the timing of diagnosis and the adequacy of long-term hormone replacement. Unrecognized neonatal salt-wasting crises can be lethal; newborn screening programs have substantially reduced neonatal mortality from this cause in populations where screening is implemented.

With appropriate glucocorticoid and, where applicable, mineralocorticoid replacement, pubertal development generally proceeds at the expected chronologic age in adequately treated individuals. Fertility rates are reported to be lower overall than the general population for both sexes; females face barriers including anatomical factors from prior genital virilization, hormonal influences on ovarian function, and psychosocial considerations. Final adult height may fall below the expected genetic target even with early treatment, reflecting the effects of androgen excess on bone maturation before or early in treatment. Genotype-phenotype correlations are imperfect and individual outcomes vary, including among individuals with identical genotypes. Males with the salt-wasting form who receive adequate treatment develop normally, while untreated males develop progressive virilization and, over time, testicular adrenal rest tissue.

Numerous active clinical trials are investigating new therapeutic approaches for classic 21-OHD CAH, including corticotropin-releasing factor receptor antagonists aimed at suppressing ACTH-driven adrenal androgen overproduction, gene therapy approaches, and interventions targeting hormonal monitoring and care coordination. Trials include both pediatric and adult populations across multiple phases. Active clinical trials for this condition are listed on ClinicalTrials.gov.