What Is It, Causes, Diagnosis and More
Editors: Alyssa Haag,Józia McGowan, DO,Kelsey LaFayette, DNP, ARNP, FNP-C
Illustrator:Jessica Reynolds, MS
Copyeditor:Stacy Johnson, LMSW
What is a micropenis?
Micropenis, also known as hypoplasia of the penis, is a rare condition of a small penis size that is often related to a broader underlying syndrome or constellation of symptoms. It is usually identified in newborns who have a disorder of sexual development (e.g., 5 alpha-reductase deficiency). A micropenis is typically related to low testosterone levels in utero or after birth and is diagnosed by carefully measuring the stretched penile length.
What causes a micropenis?
Micropenis is usually caused by any disruption to virilization, or development of male characteristics, in infants assigned male at birth, most commonly due to low androgen levels during development. Typical male physical characteristics development occurs during weeks eight to twelve of gestation and continues after birth. Male virilization involves the integration of delicate pathways that are driven by several essential hormones. Human chorionic gonadotropin, or hCG, released from the mother’s placenta, helps stimulate testosterone production in the uterus. Testosterone is the dominant androgen involved in forming the internal male structures. Some of the circulating testosterone is converted into dihydrotestosterone (DHT), a more potent form that further stimulates the growth of male sexual characteristics, including penile growth. After birth, hormones (e.g., gonadotropin-releasing hormone [GnRH], follicle-stimulating hormone [FSH], luteinizing hormone [LH]) are released from the brain’s hypothalamus and the pituitary gland to help maintain and regulate the development of these sexual characteristics.
Micropenis is caused by virilization, a deficiency of one or more hormones. This commonly occurs during various disorders of sexual development, including Kallmann syndrome, 5 alpha-reductase deficiency, and Prader-Willi syndrome; chromosomal abnormalities, like Down syndrome and Kleinfelter syndrome; growth hormone deficiency; and other conditions, such as hypopituitarism, X-linked hypogammaglobulinemia, Carpenter syndrome, and Cornelia de Lange syndrome.
What are the signs and symptoms of a micropenis?
A micropenis may present as the only sign in isolated cases or manifest together with other signs and symptoms related to hormonal abnormality syndromes or genetic associations. When testosterone levels are low, many other tissues may be involved, leading to other possible symptoms. For example, individuals may undergo delayed puberty, appear younger than they are, have lean muscle mass despite exercising, fail to grow facial hair, or fail to have their voice deepen. Later in life, testicular and sexual function may be impaired, possibly leading to a low sperm count and impaired fertility. Micropenis can also present with hypospadias when the opening of the urethra is located on the ventral surface of the penis instead of being placed on the head of the penis. Lastly, micropenis can cause anxiety, stress, and other social and psychological difficulties.
How is micropenis diagnosed?
A micropenis is diagnosed through accurate measurement of the stretched penile length, which is the distance from the pubic symphysis to the tip of the glans penis when the shaft is maximally stretched, and the foreskin retracted as much as possible. A diagnosis can be confirmed if it is less than 2.5 standard deviations from the average penile length of the same age. For example, a micropenis is defined as less than two and a half centimeters in term infants. Multiple measurements may be done to increase accuracy, using the average values for diagnosis.
In addition, a thorough work-up needs to be conducted to understand if the micropenis is a sign of an underlying syndrome or condition, like a genetic process or a systemic hormonal abnormality. A thorough medical history of the individual and their family is essential. For instance, some medications that may have been taken during pregnancy could have had anti-androgen effects that interfered with the virilization of the fetus. A physical examination typically follows, searching for any signs that may indicate an association with a specific condition. Apart from measuring the stretched penile length, a careful evaluation of the appearance and maturation of other sexual features, such as the scrotal sac, the penile shaft, and the urethral meatus, is performed. To further investigate the underlying cause, laboratory testing of different hormone levels (e.g., testosterone, LH, FSH); radiological imaging, such as pelvic ultrasound to evaluate the gonads and internal genital structures, or brain MRI to assess the pituitary gland; as well as a karyotype to assess for numeral or structural chromosomal abnormalities may also be performed.Lastly, some differential diagnoses may be considered before diagnosing micropenis, which includes cases of ‘inconspicuous penis,’ referring to a group of conditions that make the penis look diminutive. In reality, it is average-sized but is concealed by different tissues, such as loose penile skin. For example, ’buried penis’ refers to excessive fatty tissue hiding part of the penis, and ‘trapped penis’ occurs when scar tissue after trauma or circumcision entraps the penis. Micropenis may also be distinguished from a webbed penis, which refers to a skin tissue connection between the penis and the scrotum, and other rare conditions, such as penile agenesis (i.e., absence of penis) or chordee (i.e., impaired curvature of the head of the penis).
What is the treatment for micropenis?
Each case of micropenis is uniquely considered, but treatment typically consists of an initial hormone therapy approach chosen based on the associated syndrome. Testosterone therapy is most commonly given either intramuscularly (IM) or with a topical cream. These treatments aim to establish normal sexual and urinary function and minimize the social shame and discomfort often associated with micropenis. A surgical penile reconstruction is an additional option if medical treatment does not obtain the desired outcomes.In rare and particular cases of disorders of sexual development involving ambiguous genitalia, the individual may request medical advice concerning gender reassignment. In any case, micropenis and its associated disorders may be medically reviewed and managed by a multidisciplinary team consisting of a urologist, a pediatric endocrinologist, and other specialists, such as geneticists, psychologists, and social workers.
What are essential facts to know about micropenis?
A micropenis, also known as hypoplasia of the penis, is a rare condition characterized by small penis size. Diagnosis consists of carefully measuring the length of a stretched penis and comparing it to the average penis size according to age. A further investigation of the possible association with related syndromes, such as disorders of sexual development and sex hormones, may also be conducted. Since the ultimate cause of micropenis is testosterone deficiency, testosterone treatments are the most common therapeutic approach, but a surgical approach may also be necessary for some instances. A multidisciplinary approach is typically taken with management and is usually accompanied by a psychological support plan.
Watch related videos:
Androgens and antiandrogens
Disorders of sexual development and sex hormones: Pathology review
Resources for research and reference
Alsaleem, M., Saadeh, L. Micropenis. [Updated 2021 Nov 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562275/
Chan, Y.-M., & Levitsky, L. L. (2021, February 18). Evaluation of the infant with atypical genital appearance (difference of sex development). UpToDate. Retrieved May 29, 2022, from https://www.uptodate.com/contents/evaluation-of-the-infant-with-atypical-genital-appearance-difference-of-sex-development
Hatipoğlu, N., Kurtoğlu, S. (2013). Micropenis: etiology, diagnosis, and treatment approaches. Journal of clinical research in pediatric endocrinology, 5(4), 217–223. https://doi.org/10.4274/Jcrpe.1135
Houk, C. P., Baskin, L. S., Levitsky, L. L., & Hoppin, A. G. (2022, October 15). UpToDate. Retrieved June 1, 2022, from https://www.uptodate.com/contents/management-of-the-infant-with-atypical-genital-appearance-difference-of-sex-development
Synder, P. J. (2022, June 1). Clinical features and diagnosis of male hypogonadism. UpToDate. Retrieved May 5, 2022, from https://www.uptodate.com/contents/clinical-features-and-diagnosis-of-male-hypogonadismSrinivasan, A. K., Palmer, L. S., & Palmer, J. S. (2011). Inconspicuous penis.
TheScientificWorldJournal, 11, 2559–2564. https://doi.org/10.1100/2011/238519