Testosterone Replacement Therapy (TRT) Explained: What It Is, Who Actually Needs It, and What the Evidence Shows

Testosterone replacement therapy has moved from a niche medical treatment for men with serious hormonal disorders to one of the fastest-growing prescriptions in American medicine. TRT prescriptions increased more than 300% in the decade between 2001 and 2011, and have continued growing since. Testosterone clinics and men’s health telehealth platforms now offer it with minimal gatekeeping.

This expansion reflects two things happening simultaneously: a genuine and previously underdiagnosed condition (hypogonadism) being better recognized and treated, and a commercial industry aggressively marketing testosterone to men with normal age-related testosterone variation who don’t meet clinical criteria for hypogonadism.

Understanding the difference between these two populations – and what the evidence actually shows about benefits and risks – matters enormously. TRT is a legitimate and life-changing treatment for men with true hypogonadism. It’s a medication with real risks when used in men who don’t need it.

What Testosterone Actually Does in Men

Testosterone is the primary male sex hormone, produced mainly by Leydig cells in the testes under stimulation from luteinizing hormone (LH) from the pituitary. Normal male total testosterone ranges from approximately 300-1000 ng/dL, with levels typically peaking in the early 20s and declining at approximately 1-2% per year thereafter.

Testosterone’s physiological roles in adult men:

• Sexual function: Libido, erectile function, and sperm production all require adequate testosterone
• Muscle mass and strength: Testosterone is the primary anabolic hormone; it drives muscle protein synthesis and inhibits muscle protein breakdown
• Bone density: Testosterone (partly through conversion to estradiol) maintains bone mineral density; low testosterone accelerates bone loss
• Body composition: Testosterone promotes lean mass and inhibits fat accumulation, particularly visceral fat
• Red blood cell production: Testosterone stimulates erythropoietin, increasing hemoglobin and hematocrit
• Mood and cognition: Testosterone affects mood, motivation, confidence, and some cognitive functions
• Energy levels: Fatigue is one of the most consistent symptoms of low testosterone

What Hypogonadism Actually Is

Hypogonadism is the clinical condition of inadequate testosterone production. It’s classified as:

Primary hypogonadism: The testes are failing to produce adequate testosterone despite normal pituitary stimulation. LH and FSH are elevated (the pituitary is “trying harder”). Causes include Klinefelter syndrome (47,XXY karyotype), testicular trauma or infection, chemotherapy/radiation damage, cryptorchidism, and autoimmune conditions.

Secondary hypogonadism: The pituitary or hypothalamus isn’t providing adequate stimulation to otherwise functional testes. LH and FSH are low or inappropriately normal. Causes include pituitary tumors or damage, hyperprolactinemia, obesity (adipose tissue converts testosterone to estrogen, suppressing the HPT axis), obstructive sleep apnea, opioid use, anabolic steroid use (which profoundly suppresses the axis), Kallmann syndrome, and hemochromatosis.

Late-onset hypogonadism (age-related): The gradual decline in testosterone with aging that eventually falls below the clinical threshold.

The diagnostic standard: The Endocrine Society and AUA define hypogonadism as consistently low testosterone (two morning fasting measurements below 300 ng/dL) combined with symptoms. The symptoms requirement matters – a man with testosterone of 280 ng/dL and no symptoms is different from a man with the same level and significant fatigue, sexual dysfunction, and mood changes.

The Symptoms of Hypogonadism

Symptoms that may indicate low testosterone and warrant testing:

Sexual symptoms (most specific to testosterone):

• Reduced libido – often the most bothersome symptom
• Erectile dysfunction, particularly reduced morning erections
• Reduced ejaculatory volume
• Infertility

Physical symptoms:

• Fatigue and reduced energy disproportionate to activity level
• Reduced muscle mass and strength despite training
• Increased body fat, particularly abdominal
• Reduced bone density (often found incidentally on DEXA)
• Anemia (normocytic)
• Reduced body and facial hair
• Hot flashes (in more severe cases)
• Testicular atrophy

Psychological symptoms:

• Depressed mood, irritability
• Reduced motivation and drive
• Difficulty concentrating
• Reduced sense of wellbeing

The challenge: most of these symptoms are non-specific. Fatigue, low mood, weight gain, and reduced libido have many causes beyond low testosterone – including depression, sleep apnea, thyroid disease, cardiovascular disease, and normal aging. Symptoms alone are insufficient for diagnosis; measurement is required.

Diagnosing Low Testosterone Properly

This is where significant variation exists between evidence-based clinical practice and the commercial testosterone clinic industry.

What proper diagnosis requires:

• Two fasting morning measurements (testosterone peaks in the morning; a single afternoon value can be misleadingly low even in men with normal testosterone)
• Total testosterone measurement (the standard starting point)
• Free testosterone in men with symptoms and borderline total testosterone – SHBG levels affect how much testosterone is biologically available; men with high SHBG may have normal total testosterone but low free testosterone
• LH and FSH to determine primary vs secondary hypogonadism
• Prolactin to screen for pituitary adenoma (an important reversible cause)
• CBC, metabolic panel, lipids for baseline and monitoring
• Evaluation for reversible causes: obesity, sleep apnea (particularly important – treating sleep apnea alone often substantially raises testosterone), opioid use, thyroid disease

What many commercial testosterone clinics don’t do:

• A single non-morning testosterone measurement
• No evaluation for reversible causes
• No LH/FSH to determine cause
• No consideration of whether lifestyle changes could normalize levels
• Prescribing to men with testosterone in the low-normal range (300-400 ng/dL) with vague symptoms

The distinction matters: a man with secondary hypogonadism from untreated sleep apnea who treats his apnea may normalize his testosterone without TRT. A man started on TRT for a reversible cause will suppress his HPT axis, potentially making the underlying problem worse and creating TRT dependence.

TRT Formulations

Intramuscular injections (testosterone cypionate or enanthate):

• Most commonly prescribed in the US
• Typically every 1-2 weeks (every 1 week produces more stable levels)
• Inexpensive (generic available)
• Produces cyclical peaks and troughs; more frequent injection reduces this

Topical gels and creams (AndroGel, Testim, Fortesta):

• Applied daily to skin; absorbed transdermally
• Produces more stable daily levels than injections
• Risk of transfer to female partners or children through skin contact (significant concern)
• More expensive than injections

Testosterone patches (Androderm):

• Applied nightly; produces relatively stable levels
• Skin irritation is common

Subcutaneous pellets (Testopel):

• Implanted under the skin every 3-6 months
• Produces stable levels without daily or weekly administration
• Minor surgical procedure for insertion; can’t easily adjust dose once implanted

Testosterone nasal gel (Natesto):

• Applied intranasally three times daily
• Less HPT axis suppression than other forms – may preserve some endogenous testosterone production and fertility

Oral testosterone (Jatenzo, Kyzarol):

• Newer formulations designed to avoid liver toxicity of older oral androgens
• Taken twice daily with food

What TRT Actually Improves – The Evidence

The clinical evidence for TRT in true hypogonadism is solid for several outcomes:

Sexual function: The strongest evidence. TRT consistently improves libido and sexual satisfaction in men with documented hypogonadism. The large TRAVERSE trial (2023) confirmed improvement in sexual desire with testosterone in older hypogonadal men.

Body composition: Testosterone reduces fat mass and increases lean mass in hypogonadal men. The effects are meaningful but not as dramatic as supraphysiological doses used in bodybuilding – TRT brings levels to normal, which produces physiological (not pharmacological) anabolic effects.

Bone density: TRT increases bone mineral density in hypogonadal men – particularly relevant for those with established osteoporosis from prolonged hypogonadism.

Mood and wellbeing: Men with documented hypogonadism report improved mood, energy, and sense of wellbeing with TRT. This is a real and clinically meaningful outcome.

Muscle strength: Modest improvement in men with hypogonadism, bringing them toward the normal range rather than above it.

What TRT does NOT clearly improve in most studies:

• Cognitive function (mixed evidence)
• Cardiovascular outcomes (see below)
• Depression in men with normal testosterone
• Fatigue in men without documented hypogonadism

The Cardiovascular Question – The Most Important Safety Issue

For several years, the cardiovascular safety of TRT was genuinely uncertain, with some studies suggesting increased risk and others suggesting benefit. The TRAVERSE trial (2023) – the largest randomized controlled trial of TRT in men at cardiovascular risk, covering 5,200 hypogonadal men with or at high risk for cardiovascular disease – provided the most definitive data:

Result: TRT did not increase the rate of major adverse cardiovascular events (heart attack, stroke, cardiovascular death) compared to placebo over a median follow-up of 33 months. This was reassuring on the primary cardiovascular safety question.

However – the trial also found:

• Atrial fibrillation: Higher rate in the testosterone group
• Pulmonary embolism (blood clots in the lungs): Higher rate in the testosterone group
• Acute kidney injury: Higher rate in the testosterone group

These findings mean TRT is not cardiovascularly neutral – it has specific risks that require consideration, particularly in men with existing cardiovascular or clotting risk.

The polycythemia risk: TRT consistently increases hematocrit (red blood cell concentration). High hematocrit increases blood viscosity and thrombosis risk. Hematocrit monitoring (every 3-6 months) and dose reduction when hematocrit becomes elevated are essential parts of proper TRT management.

TRT and Fertility: A Critical Consideration

This is the piece most commonly not discussed in commercial testosterone clinics, and it’s critically important for younger men.

TRT suppresses the HPT axis – just like anabolic steroids. LH and FSH fall to near zero, and the testes stop producing sperm (azoospermia is common with TRT). TRT is essentially a form of male contraception.

For men who want to preserve fertility:

• TRT is generally contraindicated, or used with concurrent strategies to maintain sperm production
• hCG (human chorionic gonadotropin) – which mimics LH – can be used alongside TRT to maintain some testicular testosterone production and sperm production
• Clomiphene (a SERM that stimulates LH and FSH) can raise testosterone in men with secondary hypogonadism while preserving fertility – though it’s not FDA-approved for this indication
• Nasal testosterone gel (Natesto) suppresses the HPT axis less than other formulations

The recovery issue: After stopping TRT, HPT axis recovery typically occurs over weeks to months. However, particularly in men who have been on TRT for years, recovery may be prolonged or incomplete. This is a fertility counseling point that should happen before TRT starts, not after.

TRT vs Anabolic Steroid Abuse: The Key Distinctions

The key point: the difference is dose, context, and monitoring. TRT aims to restore normal physiology; anabolic steroid abuse aims to exceed normal physiology. The health consequences differ accordingly.

Who Probably Shouldn’t Be on TRT

• Men with prostate cancer or high suspicion of prostate cancer (testosterone stimulates prostate tissue)
• Men with breast cancer
• Men with hematocrit above 50% before starting (polycythemia risk)
• Men wishing to preserve fertility without concurrent fertility protection measures
• Men with untreated or severe sleep apnea (treat sleep apnea first – it often raises testosterone substantially)
• Men with normal testosterone who have symptoms attributable to other causes

Frequently Asked Questions

My testosterone is 350 ng/dL. Should I be on TRT? 350 ng/dL is in the low-normal range. Whether this warrants TRT depends entirely on whether you have symptoms attributable to low testosterone, what your free testosterone is (SHBG matters), whether reversible causes have been addressed, and whether two morning measurements consistently confirm this level. Many men with 350 ng/dL are asymptomatic and don’t need TRT. A man with 350 ng/dL, significant sexual dysfunction, fatigue, and low free testosterone may benefit. This is a clinical conversation, not a numbers game.

Will TRT cause prostate cancer? This was a longstanding concern based on the observation that testosterone stimulates prostate tissue and that castration (removing testosterone) shrinks prostate cancer. The evidence that TRT causes prostate cancer in men without pre-existing prostate cancer is not established – the TRAVERSE trial and multiple other studies have not found increased prostate cancer incidence with TRT. However, TRT is contraindicated in men with existing or suspected prostate cancer, and PSA monitoring is standard during TRT.

How long do I need to be on TRT? For primary hypogonadism (testicular failure), TRT is typically lifelong – the testes cannot produce testosterone. For secondary hypogonadism from a reversible cause (treated sleep apnea, weight loss, stopped opioids), it may be possible to taper and discontinue with HPT axis recovery. For age-related testosterone decline, TRT is ongoing as long as symptoms and benefit warrant it. This should be a regular conversation with your prescribing provider.

Can lifestyle changes increase testosterone without TRT? Yes – and this should be tried first in men without primary hypogonadism. Weight loss (adipose tissue aromatizes testosterone to estrogen), treating sleep apnea, resistance training, optimizing sleep, reducing alcohol, and managing chronic stress all have documented positive effects on testosterone. A man who loses 40 lbs and treats his sleep apnea may bring testosterone from 280 to 450 ng/dL without TRT.

Are testosterone boosters (supplements) effective? The vast majority of over-the-counter “testosterone booster” supplements have no meaningful evidence for raising testosterone in men with normal levels or hypogonadism. Some contain ingredients with modest evidence (zinc supplementation raises testosterone in zinc-deficient men; vitamin D supplementation raises testosterone in vitamin D-deficient men) but these effects are modest and only present in men with the specific deficiency. No supplement comes close to the effect of actual testosterone.

Disclaimer

This article is for educational purposes only and does not constitute medical advice. Testosterone replacement therapy requires proper diagnosis by a qualified healthcare provider, including documented low testosterone on two morning measurements, symptom evaluation, investigation of reversible causes, and ongoing monitoring. Do not start TRT based on symptoms alone or without medical evaluation.

References

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