Physiological Basis
Hypogonadism is defined by a serum total testosterone below 300 ng/dL in conjunction with a constellation of clinical symptoms including reduced libido, erectile dysfunction, fatigue, decreased lean muscle mass, increased adiposity, depressed mood, and impaired cognitive function. The androgenic deficit disrupts multiple organ systems simultaneously. Within the corpora cavernosa, testosterone deficiency impairs smooth muscle tone, reduces endothelial nitric oxide synthase (eNOS) expression and activity, and diminishes the nitric oxide-cGMP signalling capacity that underlies physiological erection. The net effect is a haemodynamic insufficiency that compounds arteriogenic pathology and blunts response to PDE5 inhibitor therapy in men with concurrent vascular disease.
Testosterone replacement therapy restores circulating testosterone to the physiological range — most protocols target serum total testosterone between 400 and 700 ng/dL, with attention to free testosterone bioavailability in men with elevated sex hormone binding globulin (SHBG). Restored androgen levels re-establish androgen-receptor-mediated transcription in cavernosal smooth muscle cells, improve eNOS expression, and normalise the hypothalamic-pituitary-gonadal axis hormonal milieu that governs libido and sexual arousal. In men with concurrent arteriogenic ED, TRT combined with PDE5 inhibitor optimisation frequently demonstrates superior outcomes compared to either treatment alone, as testosterone deficiency independently diminishes PDE5 inhibitor responsiveness.
TRT also exerts systemic effects on erythropoiesis (stimulating red blood cell production via erythropoietin upregulation), bone mineral density, and skeletal muscle protein synthesis — all of which are relevant to overall health status in the hypogonadal patient, and all of which require monitoring as part of longitudinal TRT management.
The Treatment Protocol
TRT is delivered via three principal modalities, each with distinct pharmacokinetic profiles, practical requirements, and patient-preference considerations. Intramuscular injection of testosterone cypionate or testosterone enanthate (100–200 mg every 7 to 14 days, or micro-dosed weekly at 50–100 mg) produces predictable serum levels with a mid-cycle peak and trough pattern that can be refined with weekly micro-dosing. This is the most cost-effective modality and allows precise dose titration. Transdermal testosterone gel (1–2% formulation applied daily to the shoulder, upper arm, or abdomen) provides stable daily levels without injection, though there is a documented risk of inadvertent transfer to partners or children through skin-to-skin contact prior to the gel drying completely. Subcutaneous testosterone pellets (100–200 mg crystalline testosterone pellets, typically 6 to 12 pellets implanted in the lateral flank or buttock subcutaneous fat under local anaesthetic) provide the most physiologically stable serum testosterone levels — eliminating peak-trough variation — but require an in-office implantation procedure every 3 to 6 months and carry a higher per-cycle cost.
Monitoring is a non-negotiable component of responsible TRT management. Baseline laboratory assessment includes serum total testosterone (two morning measurements drawn before 10 AM), free testosterone, SHBG, LH, FSH, haematocrit, PSA (in men 40 years and older), and a comprehensive metabolic panel. Follow-up labs are drawn at 3 months, 6 months, and 12 months following initiation, then annually in stable patients. The treating physician adjusts dose and modality based on symptom response, trough testosterone levels, and haematocrit trends.
Who is a Candidate
Candidacy for TRT requires both biochemical confirmation and clinical symptom correlation. The Endocrine Society and American Urological Association guidelines specify that diagnosis of hypogonadism requires two separate morning serum testosterone measurements below 300 ng/dL, drawn at least 4 hours apart and ideally on different days, together with compatible clinical symptoms. Treating borderline or asymptomatic testosterone levels is not guideline-concordant and may expose patients to unnecessary risk. Men who have completed fertility planning — or who have elected fertility-preservation concurrent strategies — are appropriate candidates. Age per se is not a contraindication; late-onset hypogonadism in older men is a recognised clinical entity warranting treatment when symptomatic and biochemically confirmed.
Fertility preservation requires specific discussion before initiating TRT. Exogenous testosterone administration suppresses pituitary FSH and LH output via hypothalamic negative feedback, resulting in testicular atrophy and progressive reduction in spermatogenesis — often to azoospermia within 3 to 6 months. Men who wish to preserve fertility while correcting androgen deficiency have two primary alternatives: hCG (human chorionic gonadotropin) co-administration alongside TRT to maintain intratesticular testosterone production and testicular volume, or clomiphene citrate (a selective oestrogen receptor modulator) monotherapy, which stimulates endogenous LH and FSH secretion and elevates testosterone without suppressing spermatogenesis.
Expected Outcomes and Timeline
The symptomatic response to TRT follows a characteristic time course. Libido improvement is typically among the earliest responses, often noted within 3 to 6 weeks of initiating treatment as androgen receptor signalling in the central nervous system is restored. Erectile function improvement follows over 3 to 6 months as cavernosal nitric oxide synthesis and smooth muscle tone normalise — men with concurrent arteriogenic ED may experience delayed or partial erectile response if the vascular component is not independently addressed. Mood, energy, and cognitive improvements are reported over 3 to 12 months. Lean muscle mass gains and bone density improvements require 6 to 24 months of sustained therapy. Full assessment of TRT efficacy should not be made before 6 months of treatment at therapeutic serum levels.
Long-term TRT requires indefinite continuation to maintain effects. Cessation of therapy results in return to pre-treatment hypogonadal biochemistry and symptoms within weeks to months, as endogenous testosterone production has been suppressed. For patients who wish to discontinue TRT, a structured withdrawal protocol with close monitoring of recovery of hypothalamic-pituitary-gonadal axis function is indicated.
Safety Profile and Risks
The most clinically significant adverse effect of TRT is erythrocytosis — stimulation of erythropoiesis producing haematocrit elevation above 54%, which increases blood viscosity and theoretical thromboembolic risk. This is managed by dose reduction, modality change (injectable testosterone tends to produce more pronounced haematocrit elevation than transdermal formulations), or therapeutic phlebotomy. Testicular atrophy and suppression of spermatogenesis are predictable effects of exogenous testosterone in most men and should be discussed as expected outcomes rather than complications. Sleep apnoea exacerbation has been documented and should be assessed in at-risk patients. Injection-site reactions (bruising, pain, rare infection) are possible with intramuscular protocols; pellet extrusion or infection is a rare procedural complication with subcutaneous implantation.
Cost and Accessibility
Injectable testosterone cypionate in generic form is the most economically accessible TRT modality — medication cost alone is $50 to $150 per month, with the addition of syringes and regular laboratory monitoring. Transdermal gels carry a higher monthly medication cost, particularly in branded formulations, though generic gel preparations have reduced this differential. Subcutaneous pellet implantation involves both the pellet cost and a per-procedure physician fee for implantation, making it the highest per-cycle cost option despite less frequent administration. Some commercial insurance plans provide partial coverage for TRT medications in men with documented hypogonadism; coverage is highly variable by plan and formulary. Laboratory monitoring costs are additional and may or may not be covered depending on the specific diagnostic codes used.
Selecting a Qualified Provider
Testosterone replacement therapy for hypogonadism is best managed by a physician with formal urological or endocrinological training — specifically, a board-certified urologist with andrological expertise or an endocrinologist familiar with male hypogonadism. ABU-certified urologists with SMSNA membership are well-positioned to manage TRT within the context of comprehensive male sexual medicine care, integrating hormonal optimisation with concurrent management of erectile dysfunction, Peyronie's disease, or fertility concerns. Patients should be wary of concierge-style TRT clinics that prescribe without comprehensive diagnostic workup, do not perform guideline-mandated laboratory monitoring, or prescribe testosterone to men who do not meet biochemical and symptomatic diagnostic criteria.