From Conception to Adulthood: The Impact of Androgens on Abnormalities of Male Genital Development and Size

Abstract

Androgens play a critical role in male development starting in utero. In a 46XY male patient, testosterone plays a significant role in the development of the internal genitalia while external genitalia formation is largely dependent on the conversion of testosterone to dihydrotestosterone (DHT). Testosterone is important for testicular descent and later, the onset of puberty. Defects in the production of testosterone, the conversion of testosterone to DHT, or within the androgen receptor can lead to a variety of pediatric urological conditions including micropenis, cryptorchidism, hypospadias, and disorders of sexual differentiation. Supplementation with testosterone is the standard of care for the treatment of micropenis and is used to increase glans size and reduce complications before hypospadias repair. Unfortunately, patients with a 5-alpha reductase deficiency will respond sub-optimally to testosterone supplementation and instead may benefit from DHT supplementation.

Introduction

Androgens play an important role in male development and impact many common pediatric urological conditions. In utero, testosterone is critical for male internal genitalia development including testicular descent, and conversion of testosterone to dihydrotestosterone (DHT) is responsible for the development of external male genitalia. Postnatally, androgens lead to increased penile length during mini-puberty. During puberty, testosterone is responsible for the development of secondary sex characteristics.

Defects in testosterone production, its conversion to DHT, and the androgen receptor (AR) are the source of many of the common urological pathologies with phenotypes ranging from female in complete androgen insensitivity syndrome (CAIS) to male with hypospadias and/or micropenis. In addition, testosterone supplementation is useful in the treatment of micropenis during childhood. In this study, we discuss the involvement of testosterone and DHT in normal development and the impact of androgens in the development of common pediatric urological conditions. We also discuss the use of testosterone and DHT to treat micropenis with and without associated hypospadias.

Part I: Testosterone in normal development

Prenatal

Androgens are critical for in utero male development. In the male fetus, the testes form by 7 weeks gestation and the Sertoli cells secrete anti-Mullerian hormone (AMH), which leads to regression of the Mullerian structures. Under placental human chorionic gonadotropin (hCG) stimulation, the testicular Leydig cells produce testosterone starting at 9 weeks gestation, causing the mesonephric (or Wolffian) ducts to form the vas deferens, epididymis, and seminal vesicles.^1,2

There is a decline in placental hCG at 12–14 weeks gestation, and testosterone production becomes dependent on fetal hypothalamic–pituitary–gonadal (HPG) axis. ³ Development of male internal genitalia—epididymis, vas, seminal vesicles, and ejaculatory ducts—is facilitated by testosterone binding to the AR. Without stimulation from testosterone, the Wolffian duct structures will regress. ⁴

Development of the male external genitalia, including penile and prostatic development, is dependent on testosterone and the conversion of testosterone to DHT by type II 5-alpha reductase. Of note, both testosterone and DHT bind to the AR but DHT has much higher receptor affinity and is more potent. Masculinization of the external genitalia begins with increasing anorectal distance followed by elongation of the genital tubercle and urethral plate. By 11 weeks, the coronal sulcus is evident and by 12–16 weeks, the urethral folds have fused in the midline to form a completed penile urethra and the labioscrotal swellings fuse to form the scrotum. ² Penile growth then occurs between 20 weeks and term.

Testosterone is also involved in both phases of testicular descent. Early in development, the testis is anchored superiorly by the cranial suspensory ligament and inferiorly by the caudal genital–inguinal ligament or gubernaculum. In the first phase of descent, the cranial suspensory ligament regresses under control of testosterone. ⁵ During the second phase of descent, testosterone is responsible for testicular descent through the inguinal canal during the third trimester.^5,6 Defects in testosterone will lead to impairment or absence of both internal and external male genitalia. However, defects in DHT production alone will lead to male internal structures from local testosterone with failure of external masculinization. ²

Androgens may be responsible for other aspects of fetal development, including the higher birth weight of male compared with female infants, as males with androgen insensitivity have birth weights that are more similar to females.^3,7 Prenatal androgen exposure also influences the development of the nervous system and plays a part in defining gender roles and behavior. ³

Postnatal development

Neonatal period and mini-puberty

Although classical puberty occurs in adolescence, the HPG axis is activated in the first few months of life leading to a “mini-puberty” in which there is no change in Tanner stage. ⁸ After birth, the HPG axis is no longer suppressed by the placental steroid production, leading to a rise in gonadotropins. ⁹ During the first months of life, testosterone levels approach adult levels. However, by 6 months of age, they have returned to the prepubertal range. ¹⁰ Although no changes in Tanner staging are noted during infantile puberty, the rise in androgens is important for penile growth and testicular development. ¹¹ In particular, penile growth is correlated with testosterone levels with a significant growth period occurring during mini-puberty with an increase of 1 mm per month. ¹¹

Puberty

With the onset of puberty, the testicular volume increases to >3 mL and is associated with scrotal enlargement and scrotal skin changes.^3,12 In puberty, testosterone is produced mainly by the testes but with some contribution from the adrenal gland. ³ Testosterone secretion is tightly regulated in puberty by the HPG axis and is responsible for secondary sex characteristics seen in puberty—voice deepening and anabolic effects including the pubertal growth spurt, penile growth, and increased skeletal muscle growth. ¹³ Testosterone production increases throughout puberty but stabilizes once testicular volume reaches 15 mL. ¹⁴

Part II: Testosterone in pathological states

AR gene mutations

Although testosterone is involved in development and differentiation of the Wolffian duct, DHT binds to the AR in target tissues. ¹⁵ The AR gene is located on the X chromosome. In androgen insensitivity syndrome (AIS), there is end organ resistance to androgens, first described by Wilson et al. ¹⁶ AIS is a spectrum ranging from CAIS to partial androgen sensitivity syndrome (PAIS) in which several phenotypes are possible. CAIS patients will be phenotypic females with short blind ending vaginas.

Patients with CAIS lack Wolffian structures and do not have a prostate. On a hormonal level, CAIS patients have high testosterone and high luteinizing hormone levels, and the high testosterone is aromatized to estrogens. The clinical phenotype of PAIS patients varies from a predominately female phenotype to an under-virilized male phenotype. ¹⁷ Indeed, Kalfa et al found that among boys with isolated hypospadias, 9 of 292 boys had AR mutations; interestingly, some of the boys with AR mutations only had mild hypospadias. ¹⁸

Of note, patients with PAIS and AR mutations will have poor response to preoperative testosterone supplementation before hypospadias surgery. It is imperative that these individuals be further evaluated for PAIS or for 5-alpha reductase deficiency. Of note, topical DHT treatment has been used successfully for micropenis in two patients with PAIS with increased stretched penile length (SPL) noted after topical therapy for pre- and peripubertal males. ¹⁹ However, topical DHT therapy was not successful in the adult patient with PAIS and micropenis. ¹⁹

Micropenis

Micropenis is defined by the SPL of <2.5 standard deviations from the mean for age. ²⁰ In term infants between birth and 5 months, an SPL <2.5 cm is considered a micropenis. ²¹ Although embryological penile differentiation is complete by 12 weeks, the majority of penile growth occurs between 12 and 38 weeks in which the penile length increases by 20 mm. Therefore, micropenis can be the result of a hormonal abnormality that occurs after 12 weeks gestation. ²¹ In addition, failure of postnatal gonadotropin surge of mini-puberty can lead to decreased penile length and micropenis. ³

Micropenis originates from defects in testosterone biosynthesis or from limitation in androgen action. Examples include defects in testosterone production at any point in the HPG axis, defects in conversion of testosterone to DHT, and defects in the AR. Problems with testosterone production can be divided into hypergonadotropic hypogonadism (also called primary hypogonadism), hypogonadotropic hypogonadism, and idiopathic. ²² Idiopathic etiologies of micropenis tend to have a normal HPG axis. ²²

Isolated micropenis is often secondary to hypogonadotropic hypogonadism, usually due to a defect at the level of the pituitary or hypothalamus, and can be associated with cryptorchidism and small volume testes. It is important to include endocrinology in the management of these patients as hypogonadotropic hypogonadism can be associated with other pituitary deficiencies such as growth hormone and adrenocorticotropic hormone deficiencies. With regard to the diagnostic evaluation of micropenis outside of the mini-puberty of infancy, an hCG stimulation test is often used to identify defects in testosterone biosynthesis versus defects in androgen action.

Goals of micropenis treatment include obtaining sufficient penile size for positive body image, normal standing micturition, and sexual function. ²¹ Although there are several protocols available, the commonly utilized treatment regimen is intramuscular exogenous testosterone starting at a dose of 25 mg in infancy once monthly for 3 months. Further treatment with testosterone can be considered at the start of puberty. If patients have a good response to testosterone therapy, adult penile length can be within the normal range.^23,24

Of note, patients with micropenis who have an underlying 5-alpha reductase deficiency will have suboptimal response to testosterone therapy and instead may derive greater benefit treatment with DHT, if available. For patients who do not respond to hormonal therapy, reconstructive surgery is an option for management. Previously, sexual reassignment was considered as an option for micropenis treatment, but multiple studies have confirmed that these patients largely identify as male and sexual reassignment is no longer an acceptable option within the standard of care.^21,24

In hypospadias repair, the glansplasty can be more difficult in boys with a small glans, leading to potential poor cosmetic outcome. Like micropenis treatment, intramuscular testosterone enanthate (25 mg monthly for 3 months) has been shown to increase penile length and glans circumference without advancing the bone age or precocious puberty in patients with micropenis and hypospadias. ²⁵ Indeed, a recent prospective randomized controlled trial of intramuscular testosterone for hypospadias patients found an increased penile length and circumference in the treatment group compared with controls, and most notably, a lower surgical complication rate in the patients treated with preoperative testosterone. ²⁶

Currently, testosterone is frequently prescribed by pediatric urologists before hypospadias surgical correction. In a survey of pediatric urologists, ∼90% of high-volume surgeons used preoperative androgen therapy before hypospadias surgery. ²⁷ Although testosterone is available for supplementation in various forms including intramuscular, oral, buccal, transdermal, nasal, and subcutaneous injection for adult hypogonadal patients, the typical practice in children and infants is three doses of intramuscular testosterone. ¹²

5-Alpha reductase deficiencies

Similar to AIS, 5-alpha reductase deficiencies can lead to a spectrum of ambiguous genitalia ranging from under-virilized males to a female phenotype. Given the presence of AMH in these patients, they too will have absent Mullerian structures. These patients may also show phenotypic variation, ranging from female to male at puberty. In contrast to AIS, which is mostly commonly caused by loss of function mutations in the AR gene, the etiology of 5-alpha reductase deficiencies results from an insufficiency in type II 5-alpha reductase, which converts testosterone to DHT.

In normal development, DHT is responsible for the development of male external genitalia. Conversely, at puberty, testosterone is responsible for spermatogenesis, increased muscle mass development, and voice deepening. This explains the potential phenotype switching in individuals in which 5-alpha reductase deficiency is previously unrecognized.

Notably in the pediatric population, patients with 5-alpha reductase deficiency can present with microphallus with or without hypospadias due to low DHT synthesis during fetal development. It is thought that these boys will not respond to testosterone treatment, which is an important perioperative consideration for hypospadias patients who may have an underlying 5-alpha reductase deficiency. These patients will, however, be excellent candidates to receive DHT directly, bypassing the need for conversion.

Testosterone and DHT supplementation

Testosterone therapy is available in multiple formulations typically used for hypogonadism in the adult. Available formulations include intramuscular injection, subcutaneous injection, subdermal implants, transdermal gel, and oral/buccal. ¹² However, in pediatric practice, the majority of testosterone is given in the intramuscular form. ¹²

All testosterone supplementation is designed for adult hypogonadism, and none are specifically marketed for pediatric use. Unlike therapy for adult hypogonadism, testosterone therapy in children is aimed toward enhancing penile growth for micropenis and hypospadias, but other androgenic and anabolic effects from systemic levels of testosterone are undesirable. Most notably, testosterone supplementation should not lead to precocious puberty or changes in bone age.

Of note, transdermal testosterone therapy has been used with success in infants and children with micropenis, although the authors did note a small increase in insulin-like growth factor but no change in bone age or osteocalcin levels. ²⁸ Although topical testosterone therapy is desirable, this is not the standard of care for micropenis or preoperative hypospadias patients and is dependent on the family/caregiver for compliance.

Conversely, DHT is available as a transdermal gel that is applied directly to the target organ. Multiple treatment regimens exist but typically is 0.1–0.3 mg/kg per day for at least 3 months.^29,30 DHT has both androgenic and anabolic effects, and unlike testosterone, it cannot be aromatized to estradiol. ¹² Topical DHT has been used successfully in boys with micropenis who were found to have a mutation in 5-alpha reductase. ²⁹ Indeed, a randomized controlled trial of children with hypospadias comparing transdermal DHT with no treatment found that patients who had received DHT had a lower complication rate and improved cosmesis compared with the untreated group. ³⁰

DHT supplementation is not routinely available in the United States but is available in other countries as a 2.5% topical gel. DHT's topical formulation, which has established success in treating micropenis and the potential to treat boys with hypospadias and micropenis who may have an unknown underlying 5-alpha reductase deficiency, is an ideal alternative to testosterone for preoperative hypospadias patients.

Conclusions

Androgens play a critical role in the development of both internal and external male genitalia. Androgen abnormalities affect numerous pediatric urologic conditions and can have a wide range of phenotypes. Testosterone supplementation is useful in micropenis management as well as preoperative management of boys with hypospadias and a small glans. More recently, DHT transdermal gel supplementation can be used for boys with micropenis and 5-alpha reductase deficiency and potentially as an option for boys with micropenis and/or preoperative hypospadias treatment.