Regardless of the underlying cause of hypogonadism, pubertal induction should replicate natural physiological processes and promote the development of secondary sexual characteristics, linear growth achievement, and psychological well-being. For boys who show no signs of puberty by the age of 14 years, evaluations (such as bone age, blood examinations, and basal hormonal screening) should be conducted to determine whether pharmacological intervention is necessary. In contrast, in individuals with indicators or conditions suggestive of permanent hypogonadism (i.e., anosmia, deafness, cryptorchidism, and microphallus), pubertal induction often begins before 12 years of age or even earlier. Unfortunately, aside from rare cases in which a medical history suggestive of HH or genetic confirmation is available, no specific markers can reliably differentiate between self-limited delayed puberty (SLDP) and HH. Therefore, in individuals with delayed puberty and diagnostic uncertainty, the initiation of puberty is generally delayed from 14 years of age until late adolescence. The decision should always be tailored to suit the patient's preferences and clinical condition, in consultation with their parents [9,10].

Different testosterone formulations and treatment regimens have been suggested over the years, with many studies focusing on various strategies for using testosterone to manage HH in adults. However, there is a noticeable lack of research directly comparing these protocols; moreover, no comprehensive evidence-based guidelines have been established. This is particularly evident in children and adolescents, for whom only a limited number of formulations have been tested and even fewer were approved for inducing puberty.

In an analysis of the selected studies, intramuscular (IM) injections of testosterone esters were the preferred strategy for inducing puberty owing to their relatively low cost and simple administration. In most cases, pubertal induction has been observed with intermediate-acting testosterone esters (enanthate or cypionate) [11-14] or a mixture of long- and short-acting testosterone esters [11]. Formulations for IM injections are most commonly used in pediatric patients because the small doses needed for induction are difficult to administer with other formulations [13]. Many proposed regimens start treatment with 50–100 mg every 4–6 weeks [12,15-17] and gradually increase the dose to 50–100 mg every 6–12 months. An adult dosage of 150–250 mg/2–4 weeks is reached within 18–36 months [9,10,12,16]. Owing to their suboptimal pharmacokinetics leading to wide serum level fluctuations [14], a slow increase in therapeutic doses is advisable for a more natural mimicking of physiological puberty to maximize the growth spurt and allow psychosexual maturation and the adaptation to physical changes [16]. In contrast, puberty can be induced with a more rapid increase in testosterone levels in patients who seek medical attention in late adolescence, because achieving the final adult height is no longer a priority [10].

A long-acting IM formulation of testosterone undecanoate (TU) ester was recently developed in which the androgen is embedded in an oily compound that allows for prolonged release into the bloodstream, thereby reducing the number of injections and stabilizing serum fluctuations [14]. A 2011 study of older prepubertal adolescents by Giagulli et al. [18] showed the effectiveness of this parenteral formula in promoting pubertal development, although initial priming with oral TU was necessary to ensure a more physiologically progressive increase in testosterone levels. In the proposed regimen, after a short 3-month course with oral TU, 1,000 mg of the long-acting IM ester was administered every 6 weeks for the first 2 injections and then once every 14–12 weeks for the first 1–2 years. Compared with previous IM treatment schemes, fewer injections may improve compliance among adolescent patients, making it a more tolerable treatment. In parallel, its fixed dosage and prolonged half-life hinder the progressive rise of hematic testosterone levels with a hypothetical risk in younger boys with precocious growth plate maturation, limiting its use in older HH patients [10,14]. Therefore, this formulation could be an efficient alternative to other strategies for long-term use in adolescents and young adults after puberty.

Despite the existence of many therapeutic schemes and considerable clinical experience, especially in boys with SLDP, no universally accepted formal guidelines have been established for pubertal induction using parenteral testosterone esters in males with HH.

Implantable testosterone preparations in the form of testosterone pellets were among the earliest formulations and are effective and convenient for long-term testosterone delivery [12]. However, their use in pubertal induction has not yet been studied. Several authors agree that serum testosterone levels produced by this approach might be excessively high for the gradual androgen increase required for pubertal induction; however, it could still offer a safe and tolerable option for lifelong maintenance therapy [11,14].

Another existing testosterone formulation is oral TU, which is absorbed through the lymphatic system and bypasses liver inactivation, thus allowing the administration of smaller doses than older oral molecules. Oral TU has demonstrated efficacy for inducing the maturation of secondary sex features, although its use is limited by inconsistent oral bioavailability, a short half-life, and serum fluctuations [10-12,14]. The suggested dosage would start at 40 mg every other day for 3 months and then increase in frequency for 6–12 months before switching to higher doses of 80–120 mg daily within 18–24 months [10]. Despite being more accepted owing to its painless administration, the requirement for multiple daily doses and sensitivity to the lipid content of meals or fasting state make it an unreliable treatment strategy [11,12].

Recently, new oral TU soft capsules, which allow the solubilization and absorption of testosterone without requiring fat-rich meals, have been developed, but no studies have been conducted in prepubertal boys aged <18 years [14].

Other testosterone formulations, such as transdermal patches or gels, are widely used as replacement therapies in postpubertal hypogonadal males; however, their use in pediatric patients is extremely limited. Some experience with daily low doses of testosterone gel (2%) was derived from trials involving patients with SLDP. The benefits of gels include their noninvasive form and availability at low doses. Furthermore, transdermal absorption bypasses the first-pass metabolism, thereby reducing the risk of hepatic toxicity associated with IM formulations [10,14]. Disadvantages include low acceptance among adolescents owing to the need for daily applications and the risk of accidental transfer to others [14,19]. Thus, no such formulation has been approved for pubertal induction [11-13].

Finally, modern strategies include the use of intranasal testosterone gels and subcutaneous testosterone esters. Their effectiveness and safety as hormonal replacement therapies have been demonstrated in some trials in adults; however, no studies to date have examined pubertal induction in adolescents [10,14].

Table 1 summarizes the current approaches to using testosterone for pubertal induction in hypogonadal boys.

Several treatment strategies using exogenous gonadotropins have been developed over the past 2 decades. However, to date, only a few randomized studies have evaluated the effectiveness of this approach versus testosterone analogs, making it challenging to determine which regimen is superior or more commonly preferred [9].

Gonadotropins have been studied as stand-alone and combination treatments, with administration consistently performed via subcutaneous self-injections. FSH, responsible for Sertoli cell proliferation, is present in different preparations, including recombinant human FSH (rhFSH) and human urinary FSH (uFSH), and can be highly purified [10]. A more recent recombinant gonadotropin, corifollitropin alfa (CFA), is a proposed alternative to the above-mentioned FSH formulations [20]. Human chorionic gonadotropin (hCG), which has the biological activity of LH but a longer half-life, stimulates Leydig cells to produce endogenous testosterone and increases intratesticular testosterone levels concurrent with FSH to achieve spermatogenesis [19]. Similar to FSH, hCG is available in urinary and recombinant forms.

Unfortunately, only a few of the studies that have explored the use of gonadotropins to induce puberty were prospective multicenter trials or retrospective analyses, whereas most were reviews. As shown in Table 2, these studies differed in terms of subject age, which ranged from children to young adults, follow-up duration, and treatment approaches, including the use of hCG alone or combined with FSH.

In 2007, Raivio et al. [21] conducted a retrospective study of 14 boys (aged 9.9–17.7 years) with HH who were first treated with rhFSH alone, followed by combined rhFSH and hCG. The treatment plan involved initial prepubertal priming with rhFSH (1.5 IU/kg 3 times per week for 2 months to 2.8 years depending on the starting age), followed by pubertal induction through the addition of hCG (500–4,000 IU, 1–3 times per week). The rhFSH priming led to increased TV (0.9±0.7 mL to 1.8±1.1 mL, P<0.005) and inhibin B levels (27±14 pg/mL to 80±57 pg/mL, P<0.01), indicating the proliferation of immature Sertoli cells. Additional increases in both measures were observed in 13 patients after hCG introduction. Boys with a history of absent postnatal activation of the HPG axis had a weaker rhFSH-induced increase of TV and required longer treatment durations (≥1 year), suggesting that minipuberty plays a key role in later testicular response to gonadotropins. Despite initially small TV, 6 of the 7 boys who provided semen samples achieved spermatogenesis, highlighting the positive impact of rhFSH priming on testicular function [21].

A 2010 review by Yin and Swerdloff [12] of hypogonadism treatment in younger males reported that, in 1999, Barrio et al. [22] achieved normal sexual maturation and significant increases in TV in a group of prepubertal males with HH treated with hCG and FSH administered on alternating days.

An observational study conducted in Australia in 2012 by Zacharin et al. [23] in 2 groups of adolescents/young adults (aged 14.5–31.0 years) treated with hCG alone (500 IU twice/wk increased to 1,500 IU twice/week as puberty progressed) or combined with rhFSH (150–300 IU 3 times per week from the fourth month of the higher hCG dose). Similar Tanner-stage progression and TV increases were observed in both groups (2–8 mL to 3–30 mL, P=0.24 in group 1; 2-8 mL to 8-50 mL, P=0.11 in group 2), with no statistically significant differences in inhibin B or testosterone levels. However, a semen analysis revealed minimal evidence of spermatogenesis at 9 months in 3 of 9 patients treated with hCG alone (range, 0 to <1×10⁶/mL); in contrast, all patients who received combination therapy achieved spermatogenesis by 9 months (range, 0.2–15×10⁶/mL). This study showed convincing data that combined hCG and FSH treatment, in contrast to hCG monotherapy, in adolescents with HH can lead to testicular growth and may accelerate spermatogenesis within a time frame comparable to that of normal puberty [22].

A further investigation of this treatment strategy was conducted by Rohayem et al. [24] in 2017 in a relatively large group of adolescents with HH from 26 pediatric endocrinology centers throughout Germany. The patients were divided into 2 groups based on whether they had previously received testosterone replacement therapy. Testosteronenaïve adolescents received low starting doses of hCG (250– 500 IU twice weekly) in increments of 250–500 IU every 6 months (maximum 2,500 IU/3 times a week), and rhFSH (75–150 IU/3 times a week) was added once serum testosterone reached the targeted pubertal level (1.5 ng/mL). Boys previously treated with testosterone who had already completed pubertal virilization and linear growth received an initial full adult starting dose of hCG in 6-month increments (range, 1,500 IU 2 times weekly to maximum 2,500 IU 3 times weekly), with the addition of the same regimen of rFSH after 3 months of therapy.

This treatment led to a substantial increase in final bi-TV (BTV) in both protocol groups (5±5 mL to 34±3 mL vs. 5±3 mL to 32±3 mL) and the induction of spermatogenesis in the vast majority of patients who provided semen samples (91% vs. 95%). This study adds further clinical data to support previous findings that hCG monotherapy promotes testicular growth and spermatogenesis. However, combining hCG with FSH yielded significantly improved outcomes regardless of prior testosterone replacement therapy or underlying cause. The best results were observed in boys with acquired versus congenital HH [24].

In this regard, a 2021 retrospective analysis by Cangiano et al. [24] of 19 adolescents with congenital HH undergoing pubertal induction showed that combined uFSH (75 IU 3 times per week) and hCG (250–2,000 IU 2 to three times per week) increased BTV and induced spermatogenesis irrespective of prior testosterone priming or FSH pretreatment. In contrast, posttreatment BTV was significantly associated with cryptorchidism at birth. This likely indicates that the lack of HPG axis activation during the fetal and neonatal stages, which impedes the testes from descending, impairs the ability of the testes to respond effectively to exogenous gonadotropin stimulation later in life. Moreover, this study suggested that a positive genetic background accounting for the complete forms of congenital HH may prevent an optimal testicular response to gonadotropin stimulation.

A study from 2022 by Lambert et al. [16] of 19 patients with congenital HH reported that combined treatment with hCG (2,500–5,000 IU per week) and rhFSH (150 IU 3 times per week) could increase TV and induce spermatogenesis, with better results in boys without a history of cryptorchidism [16].

In a multicenter single-group study by Shankar et al. [20] conducted from 2017 to 2020, CFA was used with hCG to induce puberty in adolescent boys aged 14–17.9 years with HH. CFA is a recombinant gonadotropin that targets the same FSH receptor as rhFSH but has a half-life that is twice as long. This allows it to be a substitute for a 2 to 3 times per week regimen of rhFSH with just one CFA injection every 2 weeks. The proposed regimen consisted of a 12-week priming phase with subcutaneous CFA alone (100–150 μg every 2 weeks depending on body weight) followed by 52- week combined treatment with CFA at the exact dosage and hCG (500–5,000 IU twice weekly titrated according to testosterone and estradiol levels). This therapy effectively induces testicular growth and pubertal development. Although semen samples were not collected, the observed increase in inhibin B levels and decrease in anti-Müllerian hormone levels indicated that the Sertoli cells shifted from a proliferative to mature state. The authors suggested that CFA could be a viable alternative to rhFSH that offers fewer injections and potentially results in better treatment compliance. The Endo-ERN clinical practice guidelines for pubertal induction in patients with congenital pituitary hormone deficiency report that combined FSH and hCG therapy, particularly when preceded by rhFSH treatment, may be more effective than hCG alone at stimulating testicular growth and spermatogenesis [9]. However, no definitive conclusions have been reached regarding optimal treatment approaches. Accordingly, a recent systematic review and meta-analysis provided compelling evidence supporting the effectiveness of this therapy, highlighting that the combination of FSH and hCG outperformed hCG alone by increasing TV and improving spermatogenesis rates [26].

Table 3 summarizes the proposed gonadotropin regimens for pubertal induction.

Another treatment approach involves GnRH delivery in a pulsatile manner through a portable minipump. Delemarre-van de Waal [27] demonstrated that this method successfully induces virilization and testicular growth, including spermatogenesis, in male adolescents with HH. However, its high cost and troublesome administration make it an infrequently used option for inducing puberty [10].

Finally, kisspeptin-10, a neuropeptide crucial for regulating the HPG axis, has been investigated as a possible therapy for maintaining puberty in adult men with mild hypogonadism. Although this approach is still in the experimental stage and primarily limited to older patients, it may open new avenues for treatment [17].