Androgens play an indispensable role in all aspects of male somatogenesis, growth and development, and reproductive function. Testosterone (T), synthesized and secreted by testicular Loydig cells, is the main androgen supplied to the organism. In the tissues of some peripheral target organs. The steroid 5α-reductase, which is localized on the microsomal membrane of androgen target cells, relies on the coenzyme of glutathione reductase (NADPH) in erythrocytes as a hydrogen donor to convert testosterone (T) to the more biologically active dihydrotestosterone (DHT) through 5α-site reduction. Under normal physiological conditions, more than 80% of DHT in the peripheral circulation is derived from the conversion of T into peripheral target tissues by this pathway, about l5% from androstenedione, and only a small fraction is secreted directly by the testes (<5%). During male fetal sexual differentiation, DHT mediates the segregation of the urogenital sinus and the differentiation of the male genitalia in 46,xy individuals and promotes the differentiation and development of the prostate. If 5α-reductase is defective in target tissues and DHT synthesis is impaired, this results in a clinical male pseudohermaphroditism called 5α-reductase deficiency. There are two types of steroidal 5α-reductases that catalyze the conversion of testosterone to dihydrotestosterone (DHT): an alkaline PH (type I) enzyme, which is distributed in the liver and non-genital skin. The other is the acidic PH (type II) enzyme, which is mainly distributed in the external genitalia, perineal skin and prostate gland. The disease is caused by type II enzyme deficiency, so it is also known as steroid 5α-reductase 2 (SRD5A2) deficiency, which was once named pseudovaginal perineal scrotal hypospadias and familial incomplete male pseudohermaphroditism type II. Male pseudohermaphroditism caused by 5α-reductase deficiency is a rare autosomal recessive disorder. 5α-reductase is a microsomal enzyme that uses the coenzyme NADPH as a hydrogen-delivery body. 84% of the transcribed enzyme is inactive and 16% has a reduced affinity for NADPH after mutation. Although both sexes can be affected, only male patients show clinical abnormalities. The external genitalia naturally differentiate towards female in a hermaphroditic appearance, with an enlarged clitoris resembling a small penis, scrotum splitting on both sides resembling a large labial scrotum, perineal hypospadias, blind end vagina often co-opting the urethra, testes located in the inguinal canal or "large labial" scrotal fold, epididymis and vas deferens normal, prostate small and not visible. The epididymis and vas deferens are normal, and the prostate gland is small and inaccessible; the uterus, fallopian tubes, and ovaries are absent on ultrasound examination, and the prostate is absent or underdeveloped. The chromosome karyotype is 4 6, XY, and is often raised as a female at birth. 2, pubertal development puberty normal initiation, the emergence of progressive masculinization, change of voice, muscle volume increase, penis enlargement, pubic hair, axillary hair and beard growth or sparse, no acne, no temporal forehead angle hair interval receding, except for individual patients, generally no male breast development. After puberty, many patients change their sex to male. Laboratory tests 1, hormone level measurement serum testosterone T level is normal or elevated, DHT is below normal. t/DHT ratio is increased, the ratio of normal boys is 12 ± 3, 1, patients with this disease can reach more than 35. LH level is normal or mildly elevated, FSH is elevated in half of the patients. 2/Semen analysis Usually severe low sperm or no sperm, a few patients have normal sperm count. 3/ HCG excitation test When 5α-reductase deficiency is suspected in prepubertal children, human chorionic gonadotropin HCG excitation test is applied: HCG 1500U is injected intramuscularly once every other day for 3 times, revealing a gradual increase in T/DHT ratio (after HCG excitation, the reference T/DHT ratio of normal male infants within 6 months of age is 5,2±1,5, and that of boys between 6 months and 14 years of age is 11,0±4,4). 0±4,4). Differential diagnosis and treatment This syndrome should be distinguished from androgen insensitivity syndrome (AIS) caused by androgen receptor defects. The structural and functional defects of both are the more common causes of male pseudohermaphroditism. T and DHT are irreplaceable androgens in the male body: T stimulates the differentiation of male internal genital organs during male fetal sexual differentiation, while DHT stimulates the differentiation of external genital organs (penis, scrotum, male urethra) and prostate. This is due to the different sensitivity and affinity of AR on the Wolffian's duct and urogenital sinus to T and DHT in the progenitor of internal and external genitalia during sexual differentiation. In the early stage of sexual differentiation, Wolffian's duct is highly sensitive to T, and a very low concentration of T is sufficient to induce differentiation into male internal genital organs, while the AR of the urogenital sinus primordium has low sensitivity to T and low receptor capacity, and requires a high affinity for DHT in order to induce the response of masculinization. It is the most powerful androgen in the body. It is the most powerful androgen in the body. In children with 5α-reductase deficiency, DHT synthesis is impaired, and the external genitalia are abnormal, either female or blurred, but the internal genitalia are normally masculinized due to normal T synthesis, and there are no female internal genital organs. 5α-reductase activity and its androgen receptor (AR) analysis are extremely important for the etiological diagnosis of the syndrome. 5α-reductase deficiency is clinically manifested by male pseudohermaphroditism with chromosomal karyotype 4 6, XY; normal levels of T (or even high, with or without elevated LH) in serum and target tissues, but low levels of DHT and elevated T/DHT ratio. HCG stimulation test The T/DHT ratio increased even more after HCG stimulation, while the DHT level remained low. Biopsy of the skin tissue of the perineum and direct analysis of the 5α-reductase type I and II isoenzyme activity and the AR ligand binding activity of the cytoplasmic androgen receptor (cytoplasmic AR) and the intranuclear androgen receptor (intranuclear AR) of the target tissue are of diagnostic value. Androgen insensitivity syndrome (AIS) is similar to 5α-reductase deficiency, but the patient has a female body and mind, with female breast development and no male paraphilia, because the patient lacks testosterone receptors and does not respond to testosterone, but to estrogen, which is peripherally converted from testosterone. In this syndrome, male paraphilia, male personality and psychology appear at puberty, and the external genitalia develop significantly, but the prostate gland does not develop, and there is no female breast development. In androgen insensitivity syndrome (AIS), plasma T, DHT, and LH values are elevated, 5α-reductase type I and II isoenzyme activities are normal, and decreased AR ligand binding activity helps to exclude the diagnosis of AIS. Early diagnosis of the disorder and differentiation between 5α-reductase deficiency and androgen insensitivity syndrome (A1S) are essential for the treatment of the child, especially for the establishment of gender. In AIS, because high-dose androgen therapy is rarely effective, it is advocated to raise the child as a female, with female external genital orthopedics before puberty and estrogen/progestin replacement therapy during puberty and beyond. However, if children with 5α-reductase deficiency are raised as females, sexual inversion often occurs in adulthood, and their psychosexuality and sexual behavior tend to be male. Therefore, it is recommended that infants with clearly diagnosed 5α-reductase deficiency should be raised as males, undergo male external genital orthopedics before puberty, and give androgens after puberty, especially some testosterone MENT (7α-methyl- 19-nortestosterone) that does not require 5α-reduction. 19-nortestosterone), which can be used as early androgen replacement therapy.