Benefits of Testosterone Replacement and Methods of Substitution

Abstract

Testosterone substitution and replacement therapy is effective for managing testosterone deficiency. Traditional routes of administration include oral, nasal, transdermal, and intramuscular. Scrotal application of testosterone cream has been made recently available. Physician’s choice of one preparation over another is based on testosterone bioavailability, side effect profile and ability to achieve therapeutic levels. Patient’s choice is influenced by comfort, ease of use and product acceptability. This is important for compliance and achievement of good outcomes. Testosterone substitution can be overused and associated with adverse effects. Individuals at risk are older, obese with chronic cardiorespiratory disorders, and lower urinary tract symptoms. Therapeutic monitoring is vital and is achieved through measuring serum total testosterone levels and clinical follow-up. Decision on therapy outcomes should be individualised, based on symptom control and testosterone effects on organ function. Supra-therapeutic testosterone levels should be avoided as adverse outcomes such as worsening obstructive sleep apnoea, polycythaemia, and prostatic growth stimulation are more likely.

1. Introduction

In males, testosterone production occurs under the control of the hypothalamic-pituitary-testicular (HPT) axis. The gonadotropin pulse generator awakens in the lead up to puberty, driving the development of the testes, with the follicular stimulating hormone (FSH) responsible for the development of the seminiferous tubules and the luteinising hormone (LH), the production of testosterone from testicular Leydig cells. The release of gonadotropins results in enlargement of the testes, and a pair of 4 ml testes signals the start of puberty. The surge in testosterone that follows puberty is responsible for the eventual male phenotype characterised by voice deepening, body hair development, muscular and skeletal growth. Other secondary sexual characteristics include lengthening of the penis, pubic and scrotal hair. Testosterone primes the skeletal response to growth hormone (GH) stimulation and results in the growth spurt that follows puberty. Spermatogenesis arises from FSH stimulating spermatogonia development and spermatozoa production. Testosterone production is regulated via the hypothalamus-pituitary-testicular axis. Regulation of gonadotropin secretion from the gonadotroph cells in the anterior pituitary occurs through stimulation by gonadotropin-releasing hormone (GnRH) from the hypothalamus. The gonadotroph cells have testosterone (T) and estrogen (E2) receptors that receive negative inhibition from testosterone and oestradiol respectively [1].

Understanding this feedback response pathway allows the clinician to determine the cause of testicular failure. In primary testicular failure, testicular hypofunction results in the fall of testosterone levels and the consequent reduction of feedback inhibition of the hypothalamus and pituitary, result in a corresponding rise of LH and FSH. The causes of testicular hypofunction include destruction of testes through mumps, testicular cancer, alkylating chemotherapeutic agents, and orchidectomies. In secondary testicular failure, disruption of the HPT axis occurs at the hypothalamus and/or pituitary level. Some examples are large sellar tumours that compress on the pituitary, causing irreparable damage to the gonadotroph cells or damage to the pituitary after debulking of large sellar tumours. In such situations, the reduction of LH leads to very low testosterone levels. It is uncommon for direct hypothalamic injury to cause secondary testicular failure. More commonly, secondary testicular failure occurs when supraphysiological doses of testosterone are administered for prolonged periods, resulting in suppression of gonadotropins. When the supraphysiological testosterone doses are stopped, LH and FSH levels remain very low and may take months to recover.

Testosterone has an anabolic effect on cell growth and is important for skeletal and muscular development. Once biosynthesised and released by the testes, testosterone is transported in the blood, bound to sex hormone binding globulin (SHBG) with a small unbound portion (2%), as free testosterone. Around 95% of circulating testosterone reaches the target cells and exerts its effects by entering the cytoplasm and binding to the androgen receptors (ARs). The AR is widely distributed among reproductive and nonreproductive tissues, including the prostate, and seminal vesicles, external genitalia, skin, testes, cartilage, sebaceous glands, hair follicles, sweat glands, cardiac muscle, skeletal muscle and smooth muscle, gastrointestinal vesicular cells, thyroid follicular cells, adrenal cortex, liver, pineal and many cortical and subcortical regions, including spinal motor neurons [2]. The testosterone-AR complex results in rapid cellular signalling cascades via SRC/MAPK/ERK pathways that lead to cell proliferation and survival or enter the nucleus and bind to AR elements on target genes to increase gene expression. About 10% of testosterone is converted to dihydrotestosterone (DHT) via 5 α-reductase activity in the genitals, prostate, and skin. DHT is several times more potent than testosterone and is vital for the sexual differentiation of male genitalia during embryonic development [3]. Approximately 0.1% of testosterone is converted into estrogens by aromatase (CYP19A1) activity in bone, fat, or brain, where testosterone exerts its anabolic activity via the estrogen receptor (ER). All these hormones (testosterone, DHT and estrogens) exert negative feedback on the HPT axis [4].

In men, testosterone levels can vary widely, from very low levels late at night to peak levels in late morning. This is because the pituitary releases LH in a pulsatile manner, dependent upon periods of light and sleep. Normal ranges in healthy eugonadal men vary widely. This may be dependent on ethnic, genetic, and geographical factors. It is also affected by acute and chronic ill health. Total testosterone was determined in a reference sample of 456 healthy young men, (19–40 years), using Liquid Chromatography Tandem Mass Spectrometry, demonstrating that the 2.5 percentile cut-off was 12.1 nmol/l (348.3 ng/dl) and the 97.5 percentile was 41.5 nmol/l (1196.6 ng/dl). In very healthy men aged 70-89 years, the reference interval using mass spectrometry was 6.4–25.7 nmol/l (184.6-741.2 ng/dL) [5]. Men who describe hypogonadal symptoms but have testosterone levels at the lower limit of the normal eugonadal range, are thought to have functional hypogonadism. They may also have chronic disease or are obese. Clinical testosterone deficiency syndrome, or hypogonadism, refers to a spectrum of symptoms that include physical, psychological, and sexual. Hypogonadal men may report significant fatigue, and muscle weakness, as well as feelings of reduced motivation, and low mood. More commonly, sexual symptoms predominate, such as reduced libido, or disinterest in sexual relations, and poor erections. Hypogonadism is often associated with obesity in a bi-directional causal relationship. Chronically low testosterone levels may predispose to muscle atrophy and fat gain, whilst obesity is thought to exert an inhibitory effect on the gonadotrophs in the anterior pituitary and on the GnRH cells in the hypothalamus. The associations of fatigue with chronically low testosterone are multifactorial and include psychological aspects such as depression and low motivation, but also that of chronic normocytic anaemia, due to reduced erythropoiesis.

When there is clear evidence of primary or secondary testicular dysfunction, i.e., hypogonadal symptoms, associated with persistently low testosterone levels, testosterone replacement or substitution is wholly justified, as hypogonadal symptoms should respond well to intervention. However, where hypogonadal symptoms occur outside these situations, there are differences of opinion as to when testosterone therapy is initiated. An international survey of adult endocrinologists in 2015 indicated that in men with hypogonadal symptoms, 42.4% of endocrinologists surveyed would treat with testosterone replacement [6]. A placebo controlled double blinded randomised controlled trial (RCT) in >65-year-old hypogonadal men with total testosterone level below 9.5 nmol/l (275 ng/dl) showed that raising the total testosterone level to that of mid-normal range for 19–40-year-old men, significantly improved sexual function, and mood, but not vitality or walking distance [7]. Therefore, hypogonadal men with testosterone levels below 9.5 nmol/l (275 ng/dl) may be considered acceptable for testosterone therapy in the absence of correctable lifestyle factors.

2. Effects of testosterone substitution or replacement therapy

Previous observational studies have suggested that testosterone replacement therapy (TRT) may lead to increased cardiovascular risks, but recent prospective studies have not found this to be so. In contrast these studies have suggested that TRT may be beneficial in reducing the metabolic derangements associated with type 2 diabetes. In a prospective treatment study, 178 such patients with normal baseline testosterone of 12 nmol/l (346.1 ng/dl), were given testosterone undecanoate (TU) and compared with similar patients who did not choose to receive TRT. The TRT group had shown reductions in HbA1c, fasted insulin, fasted glucose levels compared to the matched group who did not receive therapy [8]. Such findings were supported in the recently published T4DM study, which had randomised obese men with the metabolic syndrome to TU depot injection compared to placebo over 6 months and found significant reductions in fasted glucose, reduced abdominal fat mass, and increased muscle mass [9].

Hypogonadal men report reduction in libido and erections, energy, strength and endurance, enjoyment of life, mood, mental concentration, and work performance. TRT is associated with improvement in these areas. The Androgen Deficiency in Ageing Males (ADAM) [10] is a research tool developed by the St. Louis University to assess hypogonadism. The International Index of Erectile Function (IIEF-5) evaluate erectile dysfunction (ED) through measures in erectile function, orgasmic function, sexual desire, intercourse satisfaction and overall sexual function [11]. Studies have shown reduction of ADAM scores and improvement in IIEF-5 scores in hypogonadal men who receive TRT.

Hypogonadal men experience significant changes to their body composition. Administration of androgen deprivation therapy for prostate cancer leads to sustained muscle loss and fat gain, most profound in the first 12 months [12]. The effects can be reversed with testosterone replacement [13], with dual energy X-ray absorptiometry (DEXA) showing reduction in fat mass and increased muscle mass. Loss of lean muscle mass is associated with ageing in men. TRT in men with sarcopenia has been associated with increase in lean muscle mass but the effects on strength and overall quality of life have been inconsistent [14]. Hypogonadal men have a significant risk of secondary osteoporosis [15]. It has been shown that in men who received androgen deprivation therapy, there is increased bone turnover and 2.5% decrease in bone mineral density annually [16]. In a subsequent sub-study of the T4DM, the T4Bone study had shown that TRT may be associated with significantly increased volumetric bone mineral density in cortical bone in obese metabolic men.

Testosterone activates the amygdala enhancing its emotional activity and its resistance to pre-frontal restraining control [17]. Aggressive behaviour arises through interaction between the amygdala and the hypothalamus which brings about emotions which the prefontal cognitive centres then modulate and control. Testosterone is important for psychological maturation of the brain during puberty, which results in anatomical and organisational changes in the male brain. Hypogonadal men often report lack of motivation and often feel low in mood. These symptoms may be due to the physical effects of low testosterone on muscle and bone function. Testosterone replacement can exert profound psychological effects. Testosterone replacement for 51 hypogonadal men (age 22–60 years) with baseline level <8.3 nmol/l (250 ng/dl) over 60 days led to significantly decreases in anger, sadness, irritability, tiredness, nervousness, and improvement in energy, friendliness, and sense of well-being [18]. The levels of testosterone and DHT correlated with improvement on initial treatment, but once therapeutic level was achieved, further escalation of testosterone no longer correlated with improvement.

3. Methods of testosterone substitution

In ancient times, ingested animal testicular extracts were known to bestow improved vitality and to act as an aphrodisiac [19]. In the early 1900s, injected testicular transplants from goats, ram, deer, boar and bulls were used and even testicles of executed criminals were used to substitute for males with sexual dysfunction [19]. From 1935, synthesis of testosterone from cholesterol became possible and led to the award of the Nobel Prize for Chemistry in 1939 [19]. Testosterone was first trialled as oral administration, but because of extensive first pass metabolism by the liver, it became necessary to alkylate the 17-hydroxy position, giving rise to androgens that are more resistant to hepatic metabolism. 17α-methyl-testosterone was introduced in 1939 by Foss and became extensively used for its androgenic and protein anabolic effects. It can be ingested orally or administered via trans buccal route [20]. After several decades of use however, it was found to cause cholestatic jaundice, reversible hepatotoxicity, and some cases of death, ending its use in the 1980s [21].

3.1 Intramuscular injections

With the problems encountered from first pass metabolism and hepatotoxicity, other alterations were made to the testosterone molecule to make testosterone administration more effective and safer. Adding an ester moiety to the 17-hydroxy position of synthetic testosterone made it hydrophobic, so it may be stored in oil and injected intramuscularly [22]. Intramuscular injections have been used from early and are currently still one of the most popular modes of administering testosterone. It results in an effective rise in testosterone levels with suppression of gonadotropins, resulting in symptomatic response. Examples are testosterone propionate, testosterone cypionate, testosterone enanthate, and TU. There are significant differences in elimination half-lives, with testosterone propionate being the shortest at 0.8 days, testosterone enanthate 4.5 days, and TU 33.9 days [23]. Testosterone propionate was first used in 1939. Administration of a single dose of 50 mg testosterone propionate led to a supraphysiological peak of 40.2 nmol/l (1159.4 ng/dL) 14 hours later, but quickly returned to below baseline on day 3 (57 hours) after injection, with the troughs between 3-7 nmol/L (86.5-201.9 ng/dL) [24]. Therefore, based on the pharmacokinetics of a single dose, testosterone propionate will have to be injected intramuscularly every 3rd day i.e. twice a week, which would not be practical for long term substitution. In addition, it is associated with peaks as high as 45 nmol/l (1297.9 ng/dL) and troughs of 3 nmol/l (86.5 ng/dL) [23]. It is now administered in a mixture of esters (propionate, phenylpropionate, isocaproate, and decanoate) marketed as Sustanon. However, it became superseded by the longer-acting testosterone enanthate.

Testosterone enanthate was first manufactured in the mid 1950s [25], and is still commonly used, popular among body builders. Snyder et al [26]. tested the efficacy of testosterone enanthate for treating male primary hypogonadism and determined that the regimens most practical in terms of injection interval and yet effective in suppressing LH to normal levels, were 200 mg every 2 weeks or 300 mg every 3 weeks. Salmimies et al administered fortnightly injections of testosterone enanthate in a range of doses from 50 to 250 mg, to hypogonadal males with a variety of aetiologies such as testicular, pituitary, or hypothalamic failure. They documented their sexual behaviour via questionnaires. In general, improvements in sexual desire, erectile frequencies and ejaculations were dose dependent. However, the treatment threshold below which hypogonadal symptoms were to occur was wide, from 6.9 nmol/l (200 ng/dL) to 15.6 nmol/l (450 ng/dL) [27]. Testosterone cypionate has a similar pharmacokinetic profile to testosterone enanthate. Intramuscular injection of 200 mg testosterone cypionate leads to a peak of 38.5 + 15.3 nmol/l (1108 + 440 ng/dl) from 2 to 3 days and progressively decline from day 5 to pre-injection baseline at 14 days [28]. There is a corresponding peak and decline in free testosterone and oestradiol levels. The frequency of injections every 2–3 weeks for both testosterone esters and the wide testosterone fluctuations makes it less than ideal for long term use.

TU was synthesised in the 1970s by adding the undecyclic acid to the C17β position of testosterone. When administered orally, it is absorbed via gut lymphatics and therefore by-passes the liver. After reaching the blood circulation, it undergoes de-esterification, releasing the unmodified testosterone [29]. It was first used orally and provided a safe non-injected method of administering testosterone that by-passed the liver and circumvented the issues of liver toxicity [30]. In 1980s, the World Health Organisation sponsored a Special Programme of Research, Development and Research Training in Human Reproduction which developed an injectable TU prepared in tea seed oil and introduced in the late 1990s. When administered as a 500 or 1000 mg dose, it resulted in a peak of 47.8 + 10.1 nmol/l (1379 + 291 ng/dL) and 54.2 + 4.8 nmol/l (1563 + 138 ng/dL) at the end of 7 days and declined to baseline by 6–8 weeks, showing an elimination half-life of 18.3 + 2.3 days and 23.7 + 2.7 days respectively [31]. It was first trialled in China with repeated 500 mg in 4 ml injections every 6–8 weeks. A newer more concentrated formulation was later developed by Jenapharm/Schering, in which 1000 mg of TU was completely dissolved in benzyl benzoate, diluted in 4 ml of refined castor oil and delivered in a vial [32].

In the initial pharmacokinetics studies carried out at Munster, Germany, a single injection of 1000 mg of TU resulted in peak testosterone level of 22.2 + 2.0 nmol/l (640 + 57.7 ng/dL) at 7–14 days, with levels >10 nmol/l (288 ng/dL) for up to 8 weeks. When 1000 mg was given every 6 weeks for 5 months, there was steady accumulation of testosterone to 40.8 + 3.8 mmol/l (1177 + 109.6 ng/dL) at 6 months. There was a significant slow and steady rise of DHT and oestradiol levels and slight decline in SHBG levels. FSH and LH were suppressed at the end of the 6-month study. There was a significant slight increase in prostate volume, but it remained low [33]. Therefore, the optimal regimen of administrating TU is to give the first two injections 6 weeks apart and thereafter, the injection interval could be 10–14 weeks depending on the trough testosterone levels [32, 33]. It was first marketed at the end of 2003 in Europe, Latin America, and in Asia as Nebido (Bayer) and in Spain and Australia as Reandron 1000 (Bayer). It is available in a 4 ml vial with a concentration of 250 mg/ml formulation. Another TU injectable formulation Aveed (Endo Pharmaceuticals Solutions Inc.) was launched in the USA in 2010 as a single 3 ml ampoule containing 750 mg of TU. The 3 ml injection was given at baseline, 4 weeks later and then maintained at 10 weeks intervals. During a multi-centre US based study the on-treatment troughs through a 64 week duration ranged from 10.8 to 13.5 nmol/l (309.6–389.8 ng/dl), and free testosterone, SHBG, DHT and oestradiol remained constant, while LH and FSH were suppressed [34].

3.2 Testosterone implants

Subdermal implant techniques were originally developed in the late 1930s and were first used to deliver testosterone pellets made up of compressed testosterone and cholesterol in the early 1950s. It has been in clinical use for >50 years, but its popularity waxed and waned during the 1980s into the 21st century. The technique and follow up procedures have been well described by Handelsman et al [35]. Implants are administered under sterile conditions by an experienced operator with an assistant, in a routine office setting using stainless steel wide-bore trocar (7.5 French gauge, 5 mm ID, 7 cm length), cannula and obturator set. Implants are placed subdermally in the lower abdominal wall lateral to or just about the level of the umbilicus. The supine patient’s abdominal skin is prepped anti-septically and draped. Following a local anaesthetic injection, a small incision is made (0.5–1.0 cm) at least 5 cm from the midline, to allow introduction of the trocar. This permits the insertion of four 200 mg testosterone pellets radiating subdermally from the initial incision site, expelled from the trocar by the obturator at 5–10 cm from the puncture site. The puncture wound is closed without suture using adhesive sterile strips and covered by a clear, waterproof adhesive dressing, left in place for 5–7 days and removed by the patient. Subsequent implantations are usually scheduled every 4–6 months, but patients are generally reviewed in-between with or without testosterone levels. Most patients have no side effects and there is a high continuation rate of >90% over a decade. Main adverse events are minor infections, and oozing or minor bleeding, and extrusion of pellets as a result. The risk predictors for adverse events are increased physical activity at work, number of pellets inserted, and thinness of patients, who make the technique more difficult because of reduced subcutaneous fat.

Jockenhovel studied the pharmacokinetics of a single subdermal implantation of T-pellets in 14 hypogonadal men with baseline testosterone level below 3.6 nmol/l (103.8 ng/dL) [36]. Following implantation, there was a rapid rise of testosterone level to about 49 nmol/l (1413.3 ng/dL) within 2 days, followed by a plateau phase for a month, and a gradual decline to baseline by 12 months. Mean serum testosterone was below 10 nmol/l (288.4 ng/dL), after 6 months. Rise in serum DHT correlated with the initial rise of serum testosterone levels, but the DHT/testosterone ratio decreased after the implantation and was significantly below baseline from day 21 to 189. SHBG demonstrated a rapid decline in response to T-pellets but returned to baseline by day 300. Both LH and FSH were rapidly and markedly suppressed by testosterone pellet implantation, recovering towards their baseline by 6 months. Again, the adverse events are low, and relatively minor, and most men are happy to continue. A potential disadvantage is the rapid and extreme peak of testosterone after implantation and its implications on prostate growth. The other consideration is a degree of training and experience is needed for the procedure. However, the pellets are relatively inexpensive.

3.3 Oral administration

Current versions are available as TU administered in softgel capsules. There are several trade names, Andriol, Jatenza, and Tlando. Clinical studies have suggested oral preparations to be effective. Serum testosterone levels have increased by more than 50% after several months of daily use. This was associated with improvement in ADAM questionnaire scores and AMS scale, but not SF-36 scale [37]. Oral TU rely on absorption via gut lymphatics and should be administered twice or thrice with meal, preferably with some fat content. Gut absorption can therefore be subjective and contributes to testosterone level fluctuations. However, it is generally a safe option for use in older men, especially if prostate specific antigen (PSA) needs to be carefully followed.

Other oral preparations include administration of 30 mg testosterone via a sustained and controlled transbuccal route [38]. This involves the application of a small tablet pressed firmly onto the gums of the upper incisors for about 30 seconds, twice daily. The adhered tablet absorbs saliva to form a gel and is subsequently absorbed into the mucosa. It leads to a steady rise of testosterone levels to between 10 nmol/L (288.4 ng/dL) and 20 nmol/l (576.8 ng/dL). Upon cessation of therapy, baseline levels are returned after 14 days. Adherence to therapy is reasonably good, with >60% compliance. However, it is not a popular mode of replacement and is not widely available in many countries except in North America. Common issues are gum mucosa irritation.

3.4 Transdermal applications

There are several methods whereby testosterone can be administered topically. These can be done transdermally via scrotal (Testoderm), or non-scrotal skin patches (Androderm) or by transdermal hydroalcoholic gel (AndroGel or Testogel), or by creams (AndroForte). Transdermal patches were first used on the scrotal skin in 1987 [39]. It was a logical choice as the scrotal skin has the thinnest stratum corneum, which would allow easy penetration. However subsequent issues with poor adhesiveness, and the need for regular shaving and excessive DHT levels, made it an unpopular choice among clinicians and patients. Non-scrotal patches were trialled [40, 41, 42], and but required an effective alcohol-based excipient to allow adequate permeation of the testosterone through the thicker stratum corneum. This resulted in frequent skin irritation and often burn-like reactions [43]. It too fell out of favour.

In 2010, the FDA had approved a unique product containing 2% w/v testosterone dissolved in a solvent mixture of ethanol and isopropanol. The testosterone solution was provided in a bottle with a metered dose pump and a detachable applicator. A metered dose delivers 1.5 ml (equivalent to 30 mg of testosterone) into the applicator, which the patient then uses to rub the solution onto the axilla, and then allow the solution to evaporate after 3 minutes. The procedure is then repeated on the other axilla. The testosterone solution also contains octisalate which aids in the permeability of transdermal testosterone. This product was effective in restoring total testosterone levels to the normal range after 15 days in most individuals, with improvement in hypogonadal symptoms, through two metered doses (60 mg) daily [44]. The men were instructed to avoid having their partners or children touch their axillary areas as traces of testosterone can be recovered after application. However, for unspecified reasons, this product was removed from the market in 2017.

Testosterone transdermal gels were introduced in the early 2000s. Over the last 10 years, the product was marketed as a hydroalcoholic gel containing 1% (10 mg/g) of testosterone, delivered via metered actuations each containing 12.5 mg of testosterone. The standard daily dose was four to eight actuations (50–100 mg), with the starting dose at 50 mg, and increased as needed in consultation with their doctors, up to 100 mg daily. Bodily sites where the patients could apply the gel include the shoulders, upper arms, chest, and upper torso areas. The gel was applied in the morning, after a shower if needed, and the applied areas allowed to dry thoroughly (within minutes) and covered with clothing appropriately to prevent inadvertent transference to others. Testosterone levels were elevated into the eugonadal range with appropriate suppression of gonadotropins and was associated with improvements in mood, sexual function, satisfaction, and body composition. Adverse events were acceptably low, at 5.5%, and were mainly from mild skin erythema [43]. Testogel has been a highly marketed product and enjoys popularity world-wide.

Testosterone is also available as a 5% (50 mg/ml) alcohol-free topical testosterone cream (AndroForte 5). The T cream was supplied with a dose measuring applicator graduated in 0.5 ml increments. Patients can start with 2 ml (100 mg) with a maximum dose of 4 ml (200 mg) of cream massaged evenly over the skin of the torso daily. The dose can be adjusted over 3 months with consultation of their doctors. The treatment is effective in raising testosterone levels into the eugonadal range with appropriate lowering of gonadotropins. It is comparable to the use of Testogel with no difference in adverse events between the two products [45]. AndroForte 5 may also be administered as a scrotal skin application. Iyer et al. have shown effective elevation of testosterone levels with just a low dose of 12.5 mg (0.25 ml) and is comparable to that of 100 mg (2 ml) applied onto the abdominal skin [46]. This suggests that an eight-fold increase in bioavailability is attained via the scrotal skin route, which as discussed, has a thin stratum corneum and also has a high concentration of 5α reductase levels, which converts testosterone to DHT levels. Patients generally start with 0.5 ml (25 mg) massaged into the scrotum daily. There is no need to remove scrotal hair as the cream is well absorbed into the scrotum. The daily dose can be increased by 0.25 ml up to 0.75 ml if needed over 3–6 months in consultation with medical practitioners. This therefore provides an economical yet effective way of testosterone substitution.

3.5 Nasal delivery

Another method of delivery that has been generally available in North America since 2015 is that of nasal testosterone gel. Natesto is a 4.5% testosterone thixotropic gel applied intranasally by delivering 5.5 mg via a metered dose pump, into each nostril two or three times a day. In an open-label dose titration study, that included overweight men (mean age 54 years) with late-onset (functional) hypogonadism, 306 were randomly assigned to either twice or thrice daily regimens [47]. Patient responses were quantified using the IIEF and the Positive and Negative Effect Schedule (PANAS) at baseline and at 30-day intervals for 3 months. Regardless of the baseline total testosterone levels, which can range from <3.4 nmol/l (<100 ng/dl) to 27.3 nmol/l (800 ng/dl), administration of Natesto led to a rapid rise of total testosterone to about 24.4–27.3 nmol/l (700–800 ng/dl) after an hour but returns to baseline after 6 hours. At day 90 of treatment, there were corresponding rises in IIEF scores of about 40%, irrespective of the baseline pre-treatment total testosterone levels and an increase in positive PANAS and decrease in negative PANAS scores. In a dose titration study, patients were started with a divided daily dose of 22 mg but may be titrated to 33 mg depending on symptoms at day 90. The majority stayed on 22 mg daily, whilst a third of patients preferred the 33 mg daily dose [48]. Symptoms were recorded using the qADAM, and patient satisfaction was documented at baseline, and at 30 day intervals up to 120 days. It has been generally well accepted with most citing convenience and ease of use, effectiveness, travel friendliness, but also listing nasal drip as a potential deterrent to using it [48].

4. Testosterone substitution in sports

Testosterone’s potent anabolic effects on musculoskeletal and bone marrow quickly led to widespread illicit use in sports [49, 50]. Testosterone propionate’s half-life of 2–3 weeks makes it popular as an ideal anabolic steroid to enhance athletic performance. It became implicated in illicit steroid use and doping scandals in sport use [51]. The advent of mandatory and sophisticated drug testing at international competitions have not eliminated the illicit use of testosterone rather, anabolic steroid use have found themselves into other lesser competitive sporting events such as in high schools and clubs [52]. Individuals have reportedly used testosterone and other anabolic steroids in a pre-specified period leading up to the competition. Giorgi et al have shown that testosterone enanthate administered at supraphysiologic doses at 3.5 mg/kg/week over 12 weeks (approximately 300 mg/week) was associated with improved physical conditioning, doubling of muscle strength and increased weight over placebo. However, upon withdrawal of testosterone injections, the enhancements did not continue, and subjects would return to their baseline 12 weeks after cessation, despite ongoing training [53]. Concerningly, youths are turning to anabolic steroids to enhance not only their physical performance but also their physical appearance [54].

5. Opioid induced androgen deficiency

Individuals who use opioids for long term are at risk of developing hypogonadism [55]. Activation of μ-receptors in the hypothalamic pituitary axis can result in suppression of the gonadal axis. The prevalence of opioid induced androgen deficiency varies from 19% to 86% depending on the testosterone threshold for defining hypogonadism. The true incidence is likely to be higher than reported as the association is under-recognised by clinicians. The risk is higher in chronic opioid users, with a Morphine Equivalent Dose of greater than 60 mg, but shorter use duration with the more potent and long-acting opioids such as fentanyl are at higher risk. On the hand, opioids with partial μ-receptor agonism such as buprenorphine or tapentadol have lower risks. Affected individuals report sexual dysfunction, depression, reduced fertility, reduced bone health, increased weight, and elevated cardiovascular risks. Testosterone replacement can be achieved via topical or parenteral routes, with benefits in reduction of pain perception, and opioid dosing, improvement in libido, body composition, and bone density.

6. Testosterone for male transgender (transmen) management

Testosterone is the key hormone for adult individuals with Gender Dysphoria, who want to transition from being a female to male. Transmen who receive testosterone substitution develop amenorrhoea, increased muscle mass and strength, male pattern body hair and deepening of voice. The US Endocrine Society recommends maintaining serum testosterone within the normal male range of 11.1–34.7 nmol/l (320–1000 ng/dl) [56]. Testosterone substitution methods have included oral, transdermal, and parenteral routes. Pelusi et al [57]. studied testosterone substitution in 45 transmen over a year and randomised them to combined T. propionate/enanthate esters intramuscular injections every 10 days, or daily testosterone gel application or T. undecanoate depot 12 weekly deep intramuscular injections. All three groups report amenorrhoea by the first year of treatment and increased lean mass and reduced fat mass. There were significant changes in lipid profiles without cardiovascular outcomes. All study subjects reported general life satisfaction at 1 year but over time, all of study subjects moved over to maintenance TU treatment. A retrospective 5-year review of 50 transmen between 21 and 42 years of age comparing T undecanoate 1000 mg IMI every 12 weeks with T enanthate 250 mg IMI every 3–4 weeks showed similar results in metabolic and anthropometric parameters. Serum testosterone rose from 1.4 + 0.6 (40.4 + 17.3 ng/dL) to 1.9 + 0.8 nmol/l (54.8 + 23.1 ng/dL) at baseline to range between 12.1 + 3.8 nmol/L (350.0 + 109.6 ng/dL) and 20.8 + 11.4 nmol/l (599.9 + 328.8 ng/dL) over the 5 years. There were consistent reports of improved life satisfaction through attainment of their desired male phenotype. Though there were significantly deranged lipid profiles, there are no reports of increased cardiovascular mortality in transmen within two decades of therapy [58].

7. Testosterone substitution pitfalls and monitoring requirements

With improved testosterone assays, the challenge of testosterone therapy is to find substitution routes that not only ensure a consistent testosterone delivery but also allow safe therapeutic monitoring and prevent transference to others. Currently the options are oral administration via the oro-gastric route, trans-buccal, transdermal gels, transdermal patches, deep intramuscular injections, and subcutaneous dermal implants. WHO recommendations for the ideal TRT preparation are that it must be safe, effective in correcting symptoms and consequences of T deficiency, inexpensive, of easy administration, with good release profile, ensuring reproducible circulating levels of T and prolonged duration of action, flexible dosing and able to maintain normal physiologic levels of testosterone. Side effects relate to the excessive peaks and troughs. They include acne, injection pain, and nerve injury. Other unintended consequences are potentially reducing male fertility, worsening sleep apnoea, increased haematocrit and precipitation of polycythaemia, prostatic hyperplasia, un-masking of undiagnosed prostatic cancer and breast cancer, the rare pulmonary oil micro-embolism (POME), and anaphylaxis.

Careful attention should be given to older men, those with prostate hypertrophy, obese men who are at risk of sleep apnoea, or unstable coronary artery disease. Testosterone therapy may unmask sleep apnoea, and if untreated, may precipitate polycythaemia. It is recommended to keep haematocrit <54%. Prostatic hyperplasia during testosterone therapy is usually not a problem if done carefully. It is prudent to start with short-acting preparations such as testosterone gels or short-acting testosterone enanthate in men with moderate risk. Consider digital rectal examination in at-risk men, for example those with family history of prostate cancer, between 55 and 69 years of age. PSA must be measured at baseline, prior to starting and 6 monthly during TRT. However, after initial 12 months of documented safety, PSA may be monitored once every 12 months. Younger or pre-pubertal males are particularly sensitive to the psychological and sexualising effects of testosterone, so it is prudent to start with low doses and to increase slowly [59]. Testosterone therapy is also contraindicated in males with hepatocyte tumours because of its stimulatory effects on Cyclin E kinase activity potentially driving carcinogenesis [60].

Patients should be counselled on the risks versus benefits of testosterone therapy. Testosterone therapy can be done effectively and safely. The various options are discussed with the patient, and the most suitable route of administration decided, based on the effectiveness, safety of monitoring, drug access, cost, and patient acceptability. Patients should be reviewed at 3–6 monthly intervals. Assessment of efficacy and side effects along with therapeutic monitoring should be done at each visit. This involves assessing testosterone levels, LH, FSH, and PSA levels. Testosterone levels often correlate with symptom improvement, but maintenance of low normal trough testosterone levels maybe important for prevention of over treatment and drug toxicity. It is particularly pertinent in long acting (depot) preparations such as intramuscular TU.

8. Non-testosterone substitution options

8.1 Tribulus terrestris

Non-testosterone herbal extracts may have a role in treating ED and male hypoactive sexual desire disorder (HSDD). Tribulus terrestris or commonly known as Puncture Vine, have been used for thousands of years in Asiatic cultures as an aphrodisiac. The plant contains steroidal saponins, flavonoids, flavanol glycosides, alkaloids, and tannins in varying quantities and proportions. These chemicals are thought to be physiologically active and are used in traditional communities for their medicinal properties. Mechanisms of action are not well understood, and may be mediated by changes in hormones, cytokines, and growth factors release, binding and action [61]. It may induce relaxation of the corpus cavernosum and therefore improve erectile function. It is proposed that saponins like dioscine, diosgenin, and protodioscin can have beneficial effects on libido [62].

Few studies have properly assessed the role of Tribulus terrestris in treating ED and HSDD in men, amid anecdotal claims of varying efficacy. A randomised double-blind, placebo-controlled trial was reported in 2016 across multiple sites in Bulgaria [63]. Patients were recruited from andrology, endocrinology and urology clinics and included males with ages >18 and <65 years, with mild to moderate ED, as defined by the IEFF score. Excluded patients were those with severe ED, spinal cord injury, primary HSDD, hyperprolactinaemia < two times upper limit, total testosterone < 8 mmol/l (230.7 ng/dl), uncontrolled major psychiatric disorders, chronic alcohol overuse, poorly controlled diabetes with HbA1c >7.0%, hypertension, unstable cardiovascular disease, stage 4 chronic kidney disease, elevated liver function tests > three times upper limit normal, prostate hypertrophy or cancer, or receiving androgen treatment. 180 patients were randomised to active group or a placebo-matched group. The active drug contained two tablets (500 mg) Tribestan administered orally three times daily after meals for 12 weeks. Tribestan (Sopharma AD) is a herbal medicinal product of Bulgaria origin, standardised with respect to furostanol saponins, calculated against protodioscin. Each tablet contains the active substance TT herba extractum siccum (35–45:1) 250 mg. The control group received identical tablet in colour and taste.

Patients were monitored monthly, with their sexual function assessed by the IIEF psychometric tool and the Global Efficacy Question (GEQ) log. Blood testing were performed at the start and end of the study and adverse events were carefully noted throughout and post study. The study showed that patients receiving Tribulus terrestris reported higher IIEF scores 2.70 OR (1.40–4.01) CI, and GEQ 4.55 OR (2.35–8.93) CI. These patients reported significantly improved erectile function, intercourse satisfaction, orgasmic function, sexual desire, and overall satisfaction. Over the course of treatment, patients receiving Tribulus terrestris did not exhibit differences in lipids, blood pressure, hormones such as total testosterone, free testosterone, SHBG, and dehydroepiandrosterone sulphate (DHEAS). Therefore, individuals with mild to moderate ED, may benefit from Tribulus terrestris without significant changes in lipids, and BP.

8.2 Selective androgen receptor modulators

As mentioned, testosterone exerts its main actions by binding to the AR, triggering important cellular signalling processes and gene transcriptions. It also has important secondary effects via aromatisation to estrogens, impacting on androgenic alopecia, bone re-modelling and by converting to DHT via 5α-reductase, exerting potent stimulatory effects on prostate growth. Since the 1990s, the concept of Selective Androgen Receptor Modulators (SARMs) as molecules that selectively activate ARs in targeted tissues without aromatisation to estrogens or undergoing 5α-reductase conversion to DHT is appealing [64]. SARMs can be engineered to specifically target AR in certain tissues while minimizing off-target effects. For example, SARMs would activate AR in muscle and bone, whilst sparing other tissues such as prostate, heart and liver. There is minimal variation in the AR structure, but the regulatory milieu of each tissue allows SARMs to possess relative tissue specificity. Upon entering the cell, SARMs bind to the AR in the cytosol, which dissociates from heat shock proteins and enters the nucleus, where the ligand-AR complex then binds with AR responsive peptides (co-activators or co-repressors) specific to certain tissues. This will influence how the SARM-AR complex may bind specific DNA sequences and lead to transcriptional regulation of androgen-responsive genes. Several experimental SARMs were available since 1998, with pre-clinical studies demonstrating positive effects on osteoporosis, cancer cachexia, and detrusor urinary incontinence. However, to date, they have not demonstrated enough efficacy or safety to pass the clinical trials [64]. Therefore at present, SARMs cannot be a substitute for managing androgen deficiency.

8.3 Estrogen antagonism

As mentioned afore, regulation of gonadotropin secretion occurs through stimulation by GnRH from the hypothalamus, and negative inhibition of the gonadotroph cells from testosterone and estrogens respectively. Selective Estrogen Receptor Modulators (SERM) work by selectively antagonising E2 receptors on the gonadotroph cells, abolishing the inhibition on gonadotropin release, and therefore elevate gonadotropins and testosterone levels. Likewise, by preventing the conversion of testosterone to oestradiol, aromatase inhibitors (AIs) block the inhibitory effect of oestradiol on hypothalamus and pituitary, thereby increasing LH-stimulated testosterone production. Estrogen antagonism is effective in normalising testosterone levels in men with functional hypogonadism [65, 66, 67, 68, 69, 70], and both SERMs and AIs increase the testosterone/estrogen ratio with the latter, significantly more so.

The use of clomiphene citrate a SERM, was studied in obese males (<40 years) with total testosterone 10.4 nmol/l (<300 ng/dl) in a randomised double-blind placebo-controlled trial over 12 weeks. That showed positive findings with significantly improved erectile function, lean and muscle mass [71]. Use of SERMs may have beneficial effects on male fertility. Obese men aged 18–60 years with functional hypogonadism were randomised to receive enclomiphene, testosterone gel or placebo in a randomised double-blind double-dummy placebo-controlled trial over 16 weeks. Enclomiphene is a trans-isomer of clomiphene with consistent estrogen antagonism. As expected, there is a similar doubling of total testosterone levels in both treatment arms from a baseline of 7.0 nmol/l (203 ng/dl) but elevation of both FSH and LH levels in enclomiphene users and suppression in testosterone gel users. This was associated with a significant 80% increase in sperm concentration in men receiving enclomiphene, whilst the testosterone users showed 50% reduction [72]. An early study also suggested the possible benefit of addition of 400 mg daily vitamin E (an antioxidant) to clomiphene, in an RCT, which resulted in significant near-doubling of sperm counts and resulted in near tripling of pregnancies compared to placebo by 6 months [73].

The effect of anastrozole, a popular AI was compared with testosterone gel and placebo in older (>70 years) overweight men with functional hypogonadism using a 12-month RCT [68]. Both AI and T improved proximal muscle strength but not hand grip strength compared with placebo. Only the T group increased gait speed. There were increases in lean body mass and reduction in fat mass in the intervention groups (AI and T) but they were not statistically significant. An earlier 12-month RCT in 2009 comparing anastrozole and placebo did not observe significantly improved strength or muscle mass in older hypogonadal men receiving anastrozole. However, it may have resulted in improvement in quality of life as there was a documented reduction in ADAM scores, although the statistical significance was not reported [74]. There are variable effects and conflicting reports on sperm count and motility. It is possible that some level of estrogenic activity may be necessary for FSH stimulation and therefore sperm production [75]. Unfortunately, an RCT comparing clomiphene and anastrozole in obese younger males (mean age 32–33 years) failed to find any significance in improved quality of life outcomes using ADAM or IIEF scores, or improved sperm characteristics [70].

Risk of venous thromboembolism (VTE) taking SERMs is 0.12% per year in the normal population but likely at least three times elevated in patients with cancer. The risks of VTE in AIs are considerably less than SERMs and may be similar to that of the general population [70]. However, the overall risk may be increased in the elderly and those with cardiovascular risk factors [76]. Use of anastrozole in older men has been associated with statistically reduced lumbar spine bone density of >80% after 12 months, whilst those on testosterone gel have preserved bone density [68]. SERMs are known to increase bone mineral density [67]. An advantage of estrogen antagonists such as SERMs and AIs is that they tend to be safer in estrogen-sensitive cancers such as hepatocellular, and prostate cancers, and are often used for male breast cancer treatment. Burnett-Bowie et al. found that the PSA levels in older men receiving anastrozole were not different to placebo, despite the substantial increase in testosterone levels. It is possible the reduction of intra-prostate estrogen levels offer protection [74].

9. Conclusion

Testosterone substitution or replacement therapy is effective for managing testosterone deficiency. Popular routes of administration include oral, transdermal, and intramuscular. Scrotal application of testosterone cream is recently made available. Physician’s choice of one preparation over another is based on testosterone bioavailability, side effect profile and ability to achieve therapeutic levels. Patient’s choice is influenced by comfort, ease of use and product acceptability. This is important for compliance and achievement of good outcomes. Testosterone replacement can be overused and associated with adverse effects. Individuals at risk are older, obese with chronic cardiorespiratory disorders, and lower urinary tract symptoms. Therapeutic monitoring is vital and is achieved through measuring serum testosterone levels and clinical follow-up. Decision on therapy outcomes should be individualised, based on symptom control and testosterone effects on organ function. Supra-therapeutic testosterone levels should be avoided as adverse outcomes such as worsening obstructive sleep apnoea, polycythaemia, and prostatic growth stimulation are more likely. Assessment of symptomatic improvement is important to justify the risks versus benefits of ongoing testosterone therapy. Dose adjustment should be individualised, accounting for the patients’ co-morbidity profile, testosterone levels, haematocrit, and PSA levels. Assessment of osteoporosis with baseline bone mineral density is important, and a progress bone mineral density assessment in 12–24 months may be useful to document benefit or lack thereof. Above all, patients must always be counselled on holistic and self-care management, and include healthy lifestyle measures, such as smoking cessation, and weight management. Non-testosterone substitution options offer alternatives to testosterone therapy, and may have some positive benefit, but are not generally considered as mainstream therapy at the moment.