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5α-reductase deficiency is a rare autosomal recessive 46,XY disorder of sexual development (DSD) caused by mutations in the SRD5A2 gene, leading to deficient conversion of testosterone to dihydrotestosterone (DHT).
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https://www.youtube.com/watch?v=hklWoAyGeMM
This results in undervirilized external genitalia at birth, but normal male internal genitalia and often marked virilization at puberty.
| Pathway | Role |
|---|---|
| Testosterone → DHT | Mediated by 5α-reductase type 2 (SRD5A2 gene) |
| DHT | Critical for development of male external genitalia, prostate, and urethra |
| Deficiency | Leads to ambiguous or female-like external genitalia, but normal Wolffian duct development (epididymis, vas deferens, seminal vesicles) due to intact testosterone action |
Genetics:
Neonatal / Childhood Presentation
| Finding | Description |
|---|---|
| Ambiguous genitalia | Clitoromegaly, bifid scrotum, hypospadias |
| Female external genitalia | In some, with no palpable gonads |
| No uterus | Due to normal AMH production by Sertoli cells |
| Testes | Usually located in inguinal canal or labioscrotal folds |
Adolescence / Puberty
| Finding | Description |
|---|---|
| Virilization | Deep voice, increased muscle mass, clitoromegaly or phallic growth, increased libido |
| No breast development | Unlike AIS |
| Gender identity shift | ~50–70% may transition to male gender role during or after puberty |
| Test | Finding |
|---|---|
| Testosterone | Normal or elevated |
| DHT | Low (key feature) |
| Testosterone:DHT ratio | ↑↑ (>20–30) (hallmark) |
| LH/FSH | May be normal or mildly elevated |
| AMH | Normal (testicular function intact) |
| Karyotype | 46,XY |