Testosterone Hormone

## What it is Testosterone is the principal androgenic and anabolic steroid hormone in human males, secreted primarily by the Leydig cells of the testes under regulation by luteinizing hormone (LH) from the anterior pituitary. The hormone was first isolated in 1935 by Ernst Laqueur from bull testes and synthesized the same year by Adolf Butenandt and Leopold Ruzicka, who shared the 1939 Nobel Prize in chemistry for the work. Therapeutic use began almost immediately. Men produce roughly 4 to 9 mg per day endogenously. Total testosterone reference ranges in healthy adult men typically span 300 to 1000 ng/dL (10.4 to 34.7 nmol/L), with diurnal variation peaking in the morning. The diagnostic threshold for hypogonadism varies across guidelines: the Endocrine Society (2018) uses 264 ng/dL on two morning measurements alongside symptoms. Symptomatic hypogonadism prevalence rises with age, from roughly 5% in men aged 30 to 39 to 20% or more by age 60 to 69. Testosterone replacement therapy (TRT) is FDA-approved for primary or secondary hypogonadism with documented symptomatic deficiency. The label requires both biochemical confirmation (low total or free T on repeat measurement) and clinical symptoms. The cultural framing has drifted substantially in the last decade: TRT is increasingly prescribed and self-prescribed in men with borderline-low or normal testosterone for performance, body composition, and quality-of-life reasons. The medical evidence base supports the hypogonadal indication strongly; the case for TRT in men with normal-range testosterone is weaker. TRT formulations are diverse. Injectable testosterone esters (cypionate, enanthate) dosed weekly or every 2 weeks at 100 to 200 mg are the most common in the US. Topical gels (AndroGel, Testim, Fortesta) deliver 50 to 100 mg per day. Buccal tablets, nasal gels, subcutaneous pellets (Testopel), and the oral undecanoate (Jatenzo, Tlando) round out the formulary. Each has tradeoffs in pharmacokinetic stability, adherence, and skin-transfer risk. ## Mechanism of action Testosterone acts via the androgen receptor (AR), a nuclear hormone receptor expressed in muscle, prostate, brain, bone, skin, and adipose tissue. Binding induces dimerization and translocation to the nucleus, where the AR complex regulates transcription of androgen-responsive genes. The result is an anabolic shift: increased muscle protein synthesis, reduced fat mass, increased bone mineral density, and modulation of erythropoiesis, libido, mood, and cognitive parameters. Two metabolites matter clinically. Aromatase converts testosterone to estradiol (E2), which mediates several beneficial effects (bone density, lipid profile, libido in part) and several adverse effects when elevated (gynecomastia, water retention, mood changes). 5-alpha reductase converts testosterone to dihydrotestosterone (DHT), a more potent AR agonist that drives effects in androgen-sensitive tissues including prostate and scalp hair follicles. Exogenous testosterone suppresses endogenous LH and FSH via negative feedback, leading to testicular atrophy and impaired spermatogenesis. This is the mechanism behind the common addition of human chorionic gonadotropin (hCG) or kisspeptin agonists to maintain testicular function during TRT, particularly in men concerned about fertility. Pharmacokinetics depend entirely on formulation. Testosterone cypionate at 100 mg IM produces a peak plasma rise to 1100 to 1500 ng/dL within 24 to 72 hours, declining over 7 to 14 days. Weekly dosing produces a sawtooth pattern that some users feel as energy and mood oscillation; biweekly dosing exaggerates this. Splitting the weekly dose into two or three subcutaneous administrations smooths the curve substantially and is increasingly popular. ## Evidence base by outcome ### Sexual function in hypogonadal men A-tier. The Testosterone Trials (Snyder 2016) randomized 790 men aged 65+ with confirmed low testosterone to TRT or placebo for 1 year. The sexual function trial reported significant improvements in sexual activity, desire, and erectile function. Meta-analyses (Corona 2014, Boloña 2007) consistently report effect sizes for libido and erectile function in hypogonadal men. The signal is robust and clinically meaningful. ### Body composition in hypogonadal men A-tier. TRT increases lean mass by 1.5 to 3 kg and decreases fat mass by 1 to 2 kg over 3 to 6 months in hypogonadal men. The Testosterone Trials physical function trial reported gait-speed improvements, though the magnitude was modest. Bhasin 1996 (the canonical dose-response trial in young eugonadal men) established that supraphysiological doses produce substantial muscle hypertrophy independent of resistance training, but the supraphysiological context does not generalize cleanly to TRT-range dosing. ### Body composition in eugonadal men (TRT for performance) B-tier. Effect sizes are smaller in men with normal-range testosterone, and the risk-benefit shifts adversely. The hypertrophic ceiling can be raised but the cardiovascular and prostate considerations apply. ### Bone mineral density A-tier in hypogonadal men. The Testosterone Trials bone trial reported improvements in spine and hip bone density. Effects are smaller than dedicated bone agents (bisphosphonates, denosumab) but meaningful in the broader frame of TRT benefits. ### Cardiovascular outcomes The long-running controversy. Vigen 2013 and Finkle 2014 reported observational signals of increased CV events in TRT users, prompting an FDA labeling change. Subsequent observational data was mixed. The TRAVERSE trial (Lincoff 2023, n=5,246 men with hypogonadism and high CV risk) randomized to TRT or placebo for a mean 22 months and reported non-inferiority on the primary composite of CV death, non-fatal MI, or non-fatal stroke. The trial found higher rates of atrial fibrillation, pulmonary embolism, and acute kidney injury in the TRT arm, but no signal of MACE elevation. This is the most rigorous randomized data on the question and substantially settles the hard cardiovascular safety concern in the indicated hypogonadal population. ### Prostate cancer A-tier on the absence of incident cancer signal in TRT trials. The historical concern (Huggins 1941, that TRT 'feeds' prostate cancer) has not been borne out in modern trials and meta-analyses in men without active cancer. Trial data do show TRT can raise PSA modestly and unmask occult disease, which warrants pre-TRT prostate evaluation. TRT is contraindicated in men with active prostate cancer or untreated severe BPH. ### Erythrocytosis A-tier. TRT raises hematocrit dose-dependently. Roughly 5 to 25% of TRT users reach hematocrit above 54%, the conventional threshold for therapeutic phlebotomy. Higher in injectable than topical formulations. Erythrocytosis is the most common reason for TRT dose reduction or temporary cessation. ### Mood and cognition B-tier. Meta-analyses report small improvements in depressed mood in hypogonadal men. The Testosterone Trials cognitive function arm reported no improvement in cognitive endpoints, weakening the case for TRT in cognitive complaints. ## Dosage and titration The most common TRT protocol in the US is testosterone cypionate 100 to 200 mg per week, intramuscular or subcutaneous. Many practitioners now favor split dosing (50 to 100 mg twice weekly) or three-times-weekly (40 to 70 mg) to smooth the PK curve. Topical gels are dosed 50 to 100 mg per day. Subcutaneous pellets last 3 to 6 months per implantation. Target total testosterone is typically the mid to upper end of the reference range (600 to 900 ng/dL) measured at trough (immediately before next dose). Free testosterone, hematocrit, hemoglobin, and PSA are checked at baseline, 3 months, 6 months, and annually thereafter. Estradiol is increasingly tracked as well, particularly in users with mood or water-retention complaints. Estradiol management is one of the dominant practical issues in TRT. The aromatase inhibitor anastrozole was widely used until roughly 2018 to suppress E2; current practice is more conservative, recognizing that low E2 is itself harmful (libido, bone density, lipid profile) and that most men do not need aromatase inhibition. Use anastrozole only in symptomatic high-E2 cases at the lowest effective dose (typically 0.25 to 0.5 mg twice weekly). For fertility-preserving TRT, hCG 250 to 500 IU two to three times weekly maintains testicular function alongside TRT. Selective estrogen receptor modulators (clomiphene, enclomiphene) are alternatives to TRT for men who want to raise endogenous testosterone without exogenous androgen, particularly when fertility is a priority. No cycling is part of standard medical TRT. The performance-oriented protocols sometimes used at the supraphysiological end (alongside other agents) involve cycling and are outside the medical TRT framing. ## Side effects and safety The most common side effects are erythrocytosis (5 to 25% of users reaching HCT above 54%), acne, oily skin, increased body hair, and modest fluid retention. Injection-site reactions occur with injectables. Topical gels carry a transfer risk to partners and children that requires careful application timing and skin coverage. Gynecomastia from estradiol elevation occurs in a minority of users, more common at higher doses and in users with higher baseline aromatase activity. Mood changes cluster at the extremes (high T can produce irritability and aggression, low E2 can produce flat affect and reduced libido). Fertility suppression is essentially universal during TRT and reverses incompletely in roughly 10 to 15% of users after discontinuation. Sperm cryopreservation before TRT is reasonable for any user with future fertility plans. Sleep apnea can be unmasked or worsened. Existing sleep apnea should be evaluated and treated before TRT escalation. The TRAVERSE trial detected higher rates of atrial fibrillation, pulmonary embolism, and acute kidney injury in TRT users. These are real if uncommon adverse effects that warrant monitoring. Legal status matters. Testosterone is Schedule III in the US (Anabolic Steroids Control Act 1990) and similarly controlled in Canada, Australia, and most of Europe. Importation, distribution outside a medical relationship, and possession without a prescription are felonies in most jurisdictions. WADA bans exogenous testosterone in competition. ## Stack interactions and timing In medical TRT, the relevant 'stacks' are pharmacological adjuncts: hCG for testicular preservation, anastrozole for E2 management when needed, and clomiphene or enclomiphene as alternatives for endogenous boost. Outside medical TRT, the broader anabolic-androgenic steroid (AAS) ecosystem exists with multi-compound stacks, and that domain is materially higher-risk and outside this framing. Pair TRT with resistance training and adequate protein (1.6 to 2.2 g/kg/day) for body-composition outcomes. Vitamin D, zinc, and omega-3 are reasonable nutritional adjuncts; the additive effect on T levels in non-deficient users is small. Timing within the week matters less than total weekly exposure for most users. Split dosing produces a flatter PK curve and more even subjective state. ## Practical notes Baseline labs before starting TRT: total and free T (two morning measurements), LH, FSH, prolactin, estradiol, CBC, CMP, lipid panel, PSA (men over 40), hematocrit. Recheck labs at 3 months, 6 months, and annually thereafter. Hold dosing or reduce if hematocrit rises above 54%; therapeutic phlebotomy is the standard intervention. Quality control on the supply side is critical. Pharmaceutical-grade cypionate from a licensed pharmacy is consistent in concentration. Underground or international product is variable in concentration and contamination risk; this is the single largest practical hazard for users self-prescribing outside medical channels. The honest framing for the use case: TRT in confirmed symptomatic hypogonadism is one of the better-evidenced hormone replacements in medicine. TRT in men with normal-range testosterone for performance reasons is a different risk-benefit calculation: the body composition and subjective effects are real but smaller, and the cardiovascular, prostate, and fertility considerations apply identically. Anyone using TRT outside a medical hypogonadism diagnosis is making a different bet than the men in the supportive trials.