Causes of Micropenis
Micropenis results from inadequate androgen stimulation during fetal development, most commonly due to hypogonadotropic hypogonadism (hypothalamic or pituitary failure), hypergonadotropic hypogonadism (testicular failure), partial androgen insensitivity, or idiopathic causes. 1, 2
Hormonal and Endocrine Causes
Hypogonadotropic Hypogonadism (Central Failure)
- Hypothalamic or pituitary dysfunction is the most common endocrine cause, resulting from inadequate gonadotropin-releasing hormone (GnRH) or luteinizing hormone (LH)/follicle-stimulating hormone (FSH) production 1, 3
- Growth hormone deficiency with associated hypopituitarism can present with micropenis 1
- Congenital pituitary gonadotropin deficiency leads to fetal testosterone deficiency during critical periods of penile development 4
- Isolated gonadotropin deficiency (Kallmann syndrome and related disorders) 2
Hypergonadotropic Hypogonadism (Primary Testicular Failure)
- Testicular regression syndrome where functional testicular tissue is lost during fetal development 2
- Partial gonadal dysgenesis with impaired testosterone production 2
- Anorchia or vanishing testis syndrome 3
Disorders of Testosterone Biosynthesis and Action
- Partial androgen insensitivity syndrome where androgen receptors respond inadequately to testosterone 1, 2
- Defects in testosterone biosynthesis including 5α-reductase deficiency preventing conversion of testosterone to dihydrotestosterone (DHT) 2
- Disorders of testosterone production pathways 3
Genetic and Syndromic Causes
- Multiple genetic syndromes are associated with hypogonadotropic hypogonadism and micropenis 1
- Chromosomal abnormalities including Klinefelter syndrome and other sex chromosome disorders 2
- Prader-Willi syndrome, Laurence-Moon-Biedl syndrome, and other syndromic conditions 3
Anatomical Considerations
True Micropenis vs. Concealed Penis
- True micropenis results from hormonal defects with stretched penile length >2.5 SD below mean but normal penile structure 5
- Concealed penis (webbed penis, congenital megaprepuce, prepubic adiposity) creates the appearance of micropenis but has normal underlying penile length 5
- Trapped penis is an iatrogenic form following circumcision complications 5
Critical Diagnostic Scenario: Congenital Adrenal Hyperplasia
A newborn with an unusually large penis and bilaterally non-palpable testes requires urgent pediatric endocrinology consultation to exclude potentially fatal congenital adrenal hyperplasia (CAH), as the infant may be genetically female (46,XX) with severe virilization. 6
- This presentation mimics micropenis evaluation but represents a life-threatening emergency 6
- Failure to diagnose CAH promptly can precipitate shock, hyponatremia, and hyperkalemia 6
- Circumcision must be postponed until full endocrine evaluation is completed 6
Idiopathic Micropenis
- Approximately 10-15% of cases have no identifiable hormonal or genetic cause despite thorough evaluation 1, 3
- These patients typically respond well to testosterone therapy 1
Common Pitfalls to Avoid
- Do not confuse concealed penis with true micropenis—concealed penis has normal underlying length and does not require hormonal evaluation 5
- Do not delay hormonal evaluation—the mini-puberty window (first 2-3 months of life) provides the most reliable hormonal assessment 7
- Do not assume bilateral non-palpable testes represent simple cryptorchidism—disorders of sexual development including CAH must be ruled out first 6, 7
- Do not order imaging before hormonal assessment—ultrasound rarely assists in decision-making and should not delay evaluation 7