HRT: Complete Guide to Testosterone Replacement Therapy

Physiology

• Over 95% of circulating testosterone comes from testicular secretion, in Leydig cells. The remaining share is not secreted as testosterone by the adrenal glands: the adrenals release precursors with negligible intrinsic androgenic potency — DHEA and androstenedione — which are converted into testosterone outside the gonads, in peripheral tissues.
• Regulation occurs through the HPG axis:

Hypothalamus → GnRH → Pituitary → LH/FSH → Testes → Testosterone → negative feedback loop. Elevated testosterone feeds back on the hypothalamus to suppress GnRH secretion and reduces the responsiveness of the anterior pituitary to GnRH.

• GnRH — hypothalamic signal stimulating the pituitary.
• LH — stimulates Leydig cells to produce testosterone from cholesterol.
• FSH — acts on Sertoli cells, which support sperm production. Fertility depends on more than FSH: spermatogenesis also requires a high intratesticular testosterone concentration. That is why prescription testosterone therapy suppresses spermatogenesis and, per the Endocrine Society, is not appropriate for men with hypogonadotropic hypogonadism who want fertility in the next 6 to 12 months.

Forms of Testosterone in Blood

Published estimates of these fractions diverge, so the numbers below are approximations from different sources rather than exact constants.

The table also omits two further binders: a smaller amount of testosterone is bound to cortisol-binding globulin and to orosomucoid.

Bioavailable testosterone = free + albumin-bound. The Endocrine Society uses the same definition: albumin-bound testosterone dissociates at the capillary level, especially in tissues with relatively long blood transit times (liver, brain), and is biologically available for action there.

Why Levels Decline

Low testosterone has two very different kinds of cause. Functional causes — obesity, medications, alcohol, sleep loss — are potentially reversible when the underlying condition is treated. Organic causes are structural or genetic diseases of the testes, hypothalamus or pituitary, and no amount of lifestyle work touches them. Which of the two it is can be established only by a doctor, with tests.

1. 01

   Organic Causes — Excluded First, Not Last

   These are the causes the Endocrine Society requires to be ruled out. The lifestyle factors listed further down do not apply to them.

   • Primary (testicular): Klinefelter syndrome, cryptorchidism, anorchia, myotonic dystrophy, orchitis, testicular trauma or torsion, testicular irradiation or other damage, some types of cancer chemotherapy, orchidectomy.
   • Secondary (hypothalamic/pituitary): hypothalamic or pituitary tumor, persistent hyperprolactinemia, iron overload syndromes (hemochromatosis), infiltrative or destructive disease of the hypothalamus or pituitary, idiopathic hypogonadotropic hypogonadism.
   • What the workup involves: in secondary hypogonadism the Endocrine Society has clinicians measure prolactin and transferrin saturation; pituitary imaging is indicated in men with panhypopituitarism, persistent hyperprolactinemia, total testosterone below 150 ng/dL (5.2 nmol/L), or symptoms of tumor mass effect.
   • For a man with a prolactinoma or hemochromatosis, sleep hygiene and plastic-free food storage are not a milder version of treatment — they are a delay in getting diagnosed.
2. 02

   Age and Genetics

   • The decline is gradual and slower than the widely repeated 1–2% per year. In the Baltimore Longitudinal Study of Aging total testosterone fell by 0.124 nmol/L per year (roughly 0.6–0.8% per year) and the free testosterone index by about 1.2% per year; in the European Male Ageing Study total testosterone fell by 0.1 nmol/L per year.
   • Free testosterone declines faster than total, because SHBG rises with age.
   • The rate varies between individuals and is higher in men with adiposity and comorbid disease — it follows weight and illness more closely than the calendar. In some men the change becomes noticeable by age 40–50.
3. 03

   Obesity and Metabolic Issues

   • Adipose tissue contains aromatase, which converts testosterone to estradiol. But the main reason total testosterone reads low in men with obesity is different: obesity lowers SHBG, and conditions that lower SHBG (obesity, type 2 diabetes, androgen use) can push total testosterone below the normal range while free testosterone often stays within it. The bioavailable fraction is therefore frequently preserved rather than reduced.
   • The association moves in both directions: weight loss was associated with a proportional increase, and weight gain with a proportional decrease, in testosterone and SHBG.
   • Insulin resistance and type 2 diabetes travel with low testosterone: about one-third of men with type 2 diabetes have low concentrations, on average 86 ng/dL lower than in controls. Describing this as "impaired hypothalamic/pituitary function" claims more certainty than the data allow — the association of SHBG and total testosterone with diabetes risk is stronger than that for free testosterone, and intervention studies have produced conflicting results.
4. 04

   Stress, Sleep, and Lifestyle

   • Chronic stress and cortisol levels disrupt the hypothalamus-pituitary-testes connection.
   • Sleep restriction lowers testosterone: one week of sleep restricted to 5 hours per night decreased daytime testosterone by 10–15%. The caveat is the study behind that number — a small convenience sample of 10 healthy men with a mean age of 24 — so it cannot be extrapolated to middle-aged and older men, or to "sleep quality" in general.
   • Sleep apnea is worth treating on its own merits, but not as a way to raise testosterone: a meta-analysis of 12 studies (388 patients) found no significant change in total testosterone with CPAP (mean difference 1.08, 95% CI −0.48 to 2.64), and its authors concluded that the hypothesis of a direct interaction between obstructive sleep apnea and testosterone is not supported; strategies other than CPAP — weight reduction above all — should be considered.
5. 05

   Endocrine Disruptors and Chemical Factors

   • Phthalates, BPA, pesticides, plastics are studied almost only in observational work, and the results are inconsistent. In a nationally representative NHANES sample of 1420 men, phthalate metabolites were not significantly associated with sex hormone concentrations among men overall; associations appeared only within age subgroups — among men aged 60 and older, high molecular weight phthalates were associated with lower total, free and bioavailable testosterone, with each doubling of DEHP linked to 7.72% lower total testosterone. Human data on BPA are even less consistent than animal data.
   • Heavy metals (lead, cadmium) and various industrial chemicals are also associated with hypotestosteronism.
   • Excessive alcohol consumption and a number of medications suppress the HPG axis. The Endocrine Society lists opioids, glucocorticoids, anabolic steroid use, and alcohol and marijuana abuse among the functional causes of secondary hypogonadism.
6. 06

   Nutrient Deficiencies and Dietary Fat

   These three nutrients are usually lumped together, but the evidence behind them is not the same.

   • Zinc: deficiency reduces testosterone, and repletion improves it. The size of the effect, however, depends on baseline zinc and testosterone levels, the dosage form, the elemental zinc dose and the duration — the systematic review gives no single percentage figure.
   • Vitamin D: a double-blind randomized controlled trial (98 men, 20,000 IU per week for 12 weeks) found no effect on total testosterone in healthy middle-aged men with normal baseline levels.
   • Magnesium: randomized trials are essentially absent.
   • The claim that these deficiencies reduce androgen sensitivity has no support behind it at all.
   • Dietary fat is not a testosterone precursor in the way it is usually presented: the cholesterol used in steroidogenesis is synthesized endogenously from acetyl-CoA, and the substrate is not limited by diet. Empirically the link is weak — a meta-analysis of 6 crossover studies (206 men) found total testosterone lower on low-fat diets (SMD −0.38, 95% CI −0.75 to −0.01, P = 0.04), with the authors themselves calling for randomized controlled trials to confirm the effect, and a corrigendum was later issued for the paper.
7. 07

   Sedentary Lifestyle and Lack of Exercise

   • Low muscle mass is not a signal to the hypothalamus and pituitary — no such mechanism exists in the literature, and a meta-analysis of short-term training studies in older men found no significant effect of resistance training on basal testosterone (SDM −0.003, 95% CI −0.330 to 0.324, P = 0.986).
   • What does hold: physical inactivity and prolonged sitting are associated with obesity, metabolic syndrome and low testosterone. The route runs through body weight and metabolism, not through an "under-stimulated" axis.

Symptoms of Testosterone Deficiency

Any symptom list here comes with two warnings attached.

First, these symptoms are non-specific. The Endocrine Society states that the symptoms and signs of testosterone deficiency are non-specific and are modified by age, comorbid illness, the severity and duration of the deficiency, variations in androgen sensitivity and previous testosterone therapy. Fatigue, reduced motivation, depressed mood, poor concentration and disturbed sleep are classified explicitly as non-specific — far more often they have other causes.

Second, low testosterone is frequently asymptomatic. Low concentrations occur regularly without any symptoms or signs, and a low level on its own does not establish a diagnosis of hypogonadism.

Symptoms with a demonstrated link to low testosterone. In the European Male Ageing Study only three sexual symptoms had a syndromic association with total testosterone below 320 ng/dL (11 nmol/L) and free testosterone below 64 pg/mL (220 pmol/L):

• Poor morning erections.
• Decreased libido.
• Erectile dysfunction.

Specific signs (Endocrine Society): incomplete or delayed sexual development; loss of body (axillary and pubic) hair; very small testes (< 6 mL).

Other suggestive signs (Endocrine Society): reduced sexual desire and activity; decreased spontaneous erections, erectile dysfunction; breast discomfort, gynecomastia; eunuchoidal body proportions; inability to father children, low sperm count; height loss, low-trauma fracture, low bone mineral density; hot flushes, sweats. Note that the recognized breast-related sign is breast discomfort or gynecomastia — "increased nipple sensitivity" and a "weak pump in the chest during workouts" appear on no professional society's list of deficiency symptoms.

Non-specific physical complaints — frequent at normal testosterone as well, and not an indicator on their own:

• Fatigue, decreased energy, constant feeling of exhaustion.
• Loss of muscle mass, decreased strength, difficulty gaining/maintaining muscle.
• Increased fat mass, especially visceral fat and belly fat.
• Quick fatigue during physical activity, reduced endurance.

Non-specific cognitive and emotional complaints — same caveat:

• "Brain fog," decreased concentration and mental clarity.
• Decreased motivation, feeling "above it all."
• Irritability, impulsiveness, conflict proneness.
• Apathy, feeling that "the best is behind," diminished drive and ambition.
• Mild depressive symptoms, decreased interest in life, hobbies, training, relationships.
• Sleep problems (daytime drowsiness + insomnia at night, early awakenings).
• Increased sensitivity to stress, anxiety, internal agitation.

How to Maintain Testosterone Levels (Lifestyle Recommendations)

If a doctor has excluded organic causes, and you don't have confirmed hypogonadism requiring prescription therapy, the first level of intervention is lifestyle. This matches the Endocrine Society's position: functional hypogonadism is potentially reversible with treatment of the underlying cause, and managing conditions such as obesity brings additional health benefits in any case.

1. 01

   Sleep and Routine

   • 7–9 hours of quality sleep per night, with fixed bedtime and wake time.
   • Minimize blue light at night, cool bedroom (18–20°C), no alcohol/energy drinks before bed.
   • Have sleep apnea evaluated and treated if present — for its own sake. Do not expect CPAP to raise testosterone: in the meta-analysis it did not change total testosterone, and the lever that moves the hormone in these patients is weight reduction.
2. 02

   Training and Movement

   • Resistance training 3–4 times per week (squats, presses, rows, compound movements) builds and maintains muscle mass — a goal in its own right. It is not a way to raise the hormone: in a meta-analysis of short-term training studies in older men, resistance training did not significantly influence basal testosterone (SDM −0.003, P = 0.986), and the acute post-workout rise is transient and does not carry over into the background level.
   • HIIT and moderate cardio support cardiovascular health, metabolism, and insulin resistance.
   • Avoid chronic overtraining and excessive volume — otherwise the effect reverses.
3. 03

   Nutrition and Nutrients

   • Moderate deficit (~−300–500 kcal) or balance where you're not starving and not accumulating visceral fat.
   • Fat: there is no basis for a specific percentage target — the "30–40% of calories" figure is not taken from any recommendation, and dietary fat is not a rate-limiting precursor for testosterone. The evidence that low-fat diets go with lower total testosterone is one meta-analysis of crossover studies whose authors themselves called for confirmatory randomized trials.
   • Protein 1.6–2.2 g/kg body weight for muscle maintenance.
   • Zinc, magnesium, vitamin D — worth correcting a documented deficiency, but not as a lever of predictable size: the "improvement of 10–20%" figure has no source behind it. In zinc deficiency, repletion raises testosterone by an amount that depends on baseline levels, form, dose and duration; vitamin D showed no effect on total testosterone in a randomized trial in men with normal baseline levels; magnesium has essentially no trial evidence.
4. 04

   Stress and Recovery Management

   • Reduction of chronic stress (breathing exercises, meditation, psychotherapy, workload distribution).
   • Regular rest days, limiting work/study without breaks, at least a "mini-vacation" every 6–8 weeks.
   • Using biohacker signals (sleep quality, mood, libido) as indicators of overtraining/overload.
5. 05

   Environmental Measures

   • Minimize use of plastic for food (microwave, hot drinks), avoid phthalate-containing products.
   • Reduce consumption of ultra-processed foods rich in endocrine disruptors and hidden sugars.
   • Treat these as cheap and harmless housekeeping, not as therapy for low testosterone: in the NHANES sample there was no significant association between phthalate metabolites and sex hormones among men overall.

   If symptoms persist, the next step is not more lifestyle effort but evaluation. Testing — including prolactin, transferrin saturation and, where indicated, pituitary imaging — is ordered and interpreted by a doctor, and any hormone therapy is a prescription decision, not a self-selected one.

Testosterone itself leaves the bloodstream very quickly: the FDA prescribing information for injectable testosterone undecanoate states that reported half-life values vary considerably in the literature, ranging from 10 to 100 minutes — at most about 1.7 hours. That is why unmodified testosterone is impractical for therapy. Esters are chemical modifications that slow the release of hormone from the oil depot. All injectable testosterone esters are prescription drugs: the formulation, the dose and the interval are chosen and titrated by a physician.

How Esters Work

• Injection: the testosterone ester is injected intramuscularly or subcutaneously.
• An oil depot forms.
• The ester gradually releases, then is hydrolyzed by esterases and delivers active testosterone.

Key Characteristics

• Half-life — the time for concentration to decrease by half. For oil esters this number usually comes from pharmacokinetic studies rather than from regulatory documents: no label (Depo-Testosterone, Xyosted) states a half-life for enanthate.
• Injection frequency — depends on half-life, but the interval printed in the label is not always the interval used in clinical practice; where the two diverge, it is stated in the section on individual esters.
• Level stability — longer esters provide a smoother curve with adequate frequency.
• Molecular weight — affects the proportion of active testosterone in the preparation.

The undecanoate share is calculated from the molecular weights given in the regulators' labels — 456.7 for testosterone undecanoate and 288.42 for testosterone — which yields 63.2%, not the 61.4% quoted earlier. The other three rows are calculated correctly.

The carrier oil (sesame, castor, MCT/miglyol) affects tolerability and absorption rate but not the total testosterone volume. For long-acting depots the oil changes the pharmacokinetics dramatically: the obsolete tea seed oil formulation of testosterone undecanoate has a half-life of about 20.9 days, while the modern castor oil depots (Nebido, Aveed) release far more slowly — the Nebido summary of product characteristics describes the release rate by a half-life of 90±40 days, and a phase I pharmacokinetic study reports 33.9±4.9 days. These castor oil depots also carry the strictest administration requirements (see Testosterone Undecanoate).

Alongside exogenous testosterone, some protocols add medications aimed at the axis, fertility, and estradiol control. All of them — hCG, anastrozole, clomiphene, tamoxifen — are prescription drugs, and in the setting of testosterone therapy they are used off-label: clomiphene, anastrozole, and tamoxifen are not approved for men by either the FDA or the EMA. The AUA guideline backs this class of use only as a conditional recommendation with Grade C evidence, and only in men with testosterone deficiency who wish to maintain fertility. Nothing below is a protocol a reader can run alone: the doses named here are the ones studied in trials or written into regulator labels, and any use is prescribed, titrated, and monitored by a physician.

Baseline Panel Before Starting TRT

• Hormonal panel:
  • Total testosterone — two morning measurements (8–10 AM), fasting, on separate days. There is no single "classical guideline threshold": the AUA uses a total testosterone below 300 ng/dL as a reasonable cut-off in support of the diagnosis, while the Endocrine Society refers to the lower limit of the normal range of the particular laboratory, or to the harmonized limit of 264 ng/dL for assays certified to the CDC standard. Across medical societies the thresholds used to define low total testosterone range from 230 to 350 ng/dL, so the number depends on the assay (LC-MS/MS or immunoassay — not every laboratory has LC-MS/MS available) and on the laboratory.
  • Free testosterone and SHBG — not for every patient. The Endocrine Society recommends measuring free testosterone when total testosterone is modestly above or below the lower limit of normal (for example, 200–400 ng/dL) and in conditions that alter SHBG. The method matters: equilibrium dialysis performed under standardized conditions, or a formula that accurately calculates free testosterone from total testosterone, SHBG and albumin. Direct tracer analog immunoassays are inaccurate and should not be used.
  • LH / FSH — for differentiating primary and secondary hypogonadism.
  • Estradiol (sensitive assay) — the AUA has estradiol measured before therapy is started in men who present with breast symptoms or gynecomastia.
  • Prolactin — to rule out hyperprolactinemia.
• Metabolic and hepatic panel:
  • Fasting glucose, HbA1c, lipid profile, ALT/AST, creatinine/BUN.
• Blood pressure — a mandatory item, not an optional one:
  • Measured at baseline and periodically thereafter. FDA-approved prescribing information for testosterone products warns that testosterone can increase blood pressure, directs prescribers to monitor blood pressure periodically — especially in men with hypertension — and states that the drug is not recommended in patients with uncontrolled hypertension.
• Hematology:
  • Hematocrit, hemoglobin, PSA (by age).
  • Hematocrit before starting: the Endocrine Society (2018) lists a hematocrit above 48% (above 50% for men living at high altitude) among the conditions for which it recommends against testosterone; the AUA advises considering withholding therapy if hematocrit exceeds 50%, until the cause of the high value is explained. 54% is not a "goal" — it is the stop point on therapy (see Red Flags).

Monitoring on Therapy

The guideline schedule is less frequent than the widely quoted "every 4–6 weeks", which neither society requires:

• Endocrine Society: testosterone 3–6 months after starting therapy, with treatment aiming to raise serum testosterone into the mid-normal range; hematocrit at baseline, 3–6 months after starting, and annually thereafter.
• AUA: testosterone every 6–12 months while on therapy.
• Blood pressure: periodically, per the FDA-approved labeling, with particular attention in men with hypertension.
• When the dose or the regimen is changed: the treating physician repeats testing at the interval he or she considers appropriate. More frequent testing is not dangerous, but it is a practice choice and must not be presented as a guideline requirement.

When to Test

The sampling time depends on the preparation, and for the common injectable esters it is not the trough:

• Testosterone enanthate or cypionate: the Endocrine Society directs measuring serum testosterone midway between injections; if the mid-interval value is above 600 ng/dL or below 350 ng/dL, the physician adjusts the dose or the injection frequency.
• Injectable testosterone undecanoate and pellets: measurement at the end of the dosing interval, just before the next injection (nadir), aiming at nadir levels in the low-mid range. This is the only situation in which the guideline titrates by the trough.
• Peak — for orientation, not for titration: 24–48 hours after the injection holds for enanthate; for cypionate, peak concentrations occur roughly 2–5 days after the injection.

A target set at the trough drags the whole rest of the interval upward: a trough of 500–800 ng/dL implies peaks well above 1000–1200 ng/dL, i.e. a systematically supraphysiological profile with an increased risk of erythrocytosis.

Red Flags

• Hematocrit above 54% — risk of thrombosis, polycythemia. The Endocrine Society stops therapy until hematocrit falls to a safe level, evaluates the patient for hypoxia and sleep apnea, and reinitiates at a reduced dose; the AUA treats a hematocrit of 54% and above as warranting intervention, with dose adjustment as first-line management.
• PSA: a confirmed rise above 1.4 ng/mL over baseline in the first 12 months, a confirmed PSA above 4.0 ng/mL, or a prostatic abnormality on digital rectal examination — urological consultation, with ultrasound and biopsy if indicated.
• Rising or uncontrolled blood pressure — testosterone can increase blood pressure and is not recommended in uncontrolled hypertension.
• Erythrocytosis symptoms: severe headaches, facial flushing, shortness of breath, leg pain.

TRT is a serious medical intervention built on prescription-only drugs, not a biohack or another "supplement." Always build your therapy together with a qualified endocrinologist or urologist-andrologist under regular monitoring: the physician chooses the regimen, titrates the dose by lab results and decides when therapy has to be paused or stopped.

Key Takeaways

Resources

• Endocrine Society clinical practice guideline on testosterone therapy in men with hypogonadism (2018) — the source of the diagnostic thresholds, the contraindication list, the dosing regimens and the monitoring schedule above. Look up the guideline document itself; a journal's home page is not the guideline. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744.
• AUA guideline on the evaluation and management of testosterone deficiency (2018) — the urological perspective: the 300 ng/dL cut-off, PSA and prostate, the fertility statements, supporting medications. Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol. 2018;200(2):423-432.
• TRAVERSE (2023), the randomized cardiovascular safety trial of testosterone replacement therapy — the source of the cardiovascular figures above. Lincoff AM, Bhasin S, Flevaris P, et al. Cardiovascular Safety of Testosterone-Replacement Therapy. N Engl J Med. 2023;389(2):107-117.
• PubMed Central — original studies and reviews; search for the documents named above instead of browsing a landing page.

Final Reminder

Individual approach > copying others' protocols.

What works for one person can be dangerous or pointless for another.

Always prioritize your own health, honest labs, and working with qualified specialists over forum "recipes" and group chats.

1. 01Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice GuidelineBhasin S. et al. — J Clin Endocrinol Metab, 2018
2. 02Evaluation and Management of Testosterone Deficiency: AUA GuidelineAmerican Urological Association, 2018
3. 03Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE)Lincoff A.M. et al. — N Engl J Med, 2023 · PMID 37326322
4. 04Depo-Testosterone (testosterone cypionate) — FDA prescribing informationFDA, 2025 labeling revision (class-wide blood-pressure warning)
5. 05Nebido 1000 mg (testosterone undecanoate) — Summary of Product Characteristicselectronic medicines compendium (emc)
6. 06Onset of effects of testosterone treatment and time span until maximum effects are achievedSaad F. et al. — Eur J Endocrinol, 2011 · PMID 21753068

This guide is for educational purposes only and is not medical advice, a prescription, or a treatment recommendation. Testosterone is a prescription medicine: decisions about therapy, dosing and monitoring belong with a qualified physician. Self-administration without medical supervision carries serious health and legal risks.