Micropenis refers to the normal appearance but the penis extension length is less than 2.5 standard deviations above the average length of the penis of the same age or the same sex development normal state. Small penis is divided into 4 categories 1. hypogonadotropic hypogonadism: the lesion is located in the hypothalamus or pituitary gland, such as Kallmann syndrome; 2. hypogonadotropic hypogonadism: the lesion is located in the testes, such as Klinefelter syndrome; 3. androgen resistance syndrome: the lesion is located in the target organ, due to 5a-reductase deficiency or androgen receptor abnormalities; 4, Idiopathic: i.e., no clear cause, but the development can be basically normal after puberty. The etiology is divided into 3 categories: 1) from hypothalamus and pituitary gland; 2) gonadal testes; 3) 5a-reductase deficiency or androgen receptor abnormalities. The first test to determine the cause should be to identify whether it is the hypothalamus, pituitary gland or testes. Hormone level measurement is a common method, with hypothalamic-pituitary-gonadal axis function testing being the most common means of determining the cause of micropenis. According to Guiyuan Zhang, for infants and children, this axis is not yet well developed and is unresponsive. The measurement of serum FSH, L H and T levels in boys by radioimmunoassay before puberty is almost not clinically relevant because of their low levels, unless an HCG stimulation test is done first to assess testicular function. The HCG stimulation test is used to test the androgen secretion function of Leydig cells. Multiple injections are commonly used: HCG 1500 U, intramuscularly, once every other day for a total of 3 injections. The results: normal testicular function can increase the testosterone level by more than two times; no response or low response is usually due to primary testicular insufficiency or absence of testes; secondary hypotestosterone patients’ response depends on the degree of hypothalamic or pituitary damage; somatic delayed puberty often shows normal response; unresponsive patients can be stimulated by multiple injections of HCG. If the blood testosterone rises after repeated HCG excitation, testicular insufficiency can be excluded. The aim of treatment for micropenis is to restore the length of the penis as much as possible, to satisfy its physiological function and to benefit physical and mental health. Endocrine therapy Endocrine therapy is currently the main treatment method, but the timing of treatment, drug selection, and the route of administration are still controversial. Drugs for the treatment of small penis 1. gonadotropins that promote T production such as HCG and GnRH; 2. T substitutes such as T; 3. T that can directly promote penile growth such as DHT (dihydrotestosterone). The gonadotropins that promote T production are currently the most widely used. In order to achieve a final increase in T levels, GnRH is preferred to directly target hypothalamic and pituitary lesions, but it is currently more expensive and not widely used, so HCG is mostly used as an alternative. At present, the more recognized HCG treatment program is: the total dose is mostly 10000-15000u, twice a week, to 6 weeks as a course of treatment, such as penile growth is not obvious can be given another course of treatment. As an alternative treatment, T is mostly used when the developmentally abnormal testes are unable to produce enough T to maintain serum T levels and to promote penile growth. Therefore, Zhang Guiyuan believes that the following principles should be followed in selecting T preparations: It is preferable to choose natural T preparations rather than alkylated compounds whose T molecular structure has been modified. It is preferable to choose T preparations that can maintain stable serum T levels and avoid those that cause rapid increases and decreases in serum T levels. In order to achieve the therapeutic goal without significant fluctuation of serum T level, Zhang Guiyuan recommends choosing testosterone undecanoate injection, as it is given once a month. Surgery Surgery is mostly used for those whose endocrine treatment is ineffective. Since the results of surgery are still unsatisfactory, it is advisable to choose surgical treatment with caution. Therefore, it is mostly considered for adult men with penis less than 4 cm in the weak state or less than 7 cm in the elongated state, and it is not recommended for infants and children. The current penile lengthening procedures mainly include: 1, the pubic arch before the penile corpus cavernosum lengthening method, penile stump extension method; 2, cut off the penile superficial suspensory ligament fat flap filling method; 3, penile stump extension method, etc.