Testosterone Physiology

Testosterone is the primary male sex hormone, produced primarily in Leydig cells of the testes (95%) with a small adrenal contribution. It is regulated by the hypothalamic-pituitary-gonadal (HPG) axis: GnRH stimulates LH release from the pituitary, which stimulates testicular testosterone production in a feedback loop.

  • Forms in circulation: Total testosterone comprises free testosterone (~2%, biologically active), albumin-bound (~54%, weakly bound and bioavailable), and SHBG-bound (~44%, tightly bound, not biologically active). Free and bioavailable testosterone are more clinically relevant than total testosterone
  • SHBG (Sex Hormone Binding Globulin): Increases with age, estrogen, thyroid hormone, and liver disease; elevated SHBG can produce symptoms of low testosterone even with normal total testosterone; obesity lowers SHBG
  • Physiological roles: Libido, sexual function, sperm production, muscle protein synthesis, bone density maintenance, red blood cell production, mood and cognitive function, body fat distribution, cardiovascular function
  • Conversion: Testosterone converts to DHT (dihydrotestosterone) via 5-alpha-reductase — drives prostate growth and male pattern baldness; and to estradiol via aromatase — important for bone health, libido, and cardiovascular function in men
  • Normal range: Total testosterone 300–1,000 ng/dL (10.4–34.7 nmol/L) in adult men; laboratory reference ranges vary; symptoms must accompany low levels for hypogonadism diagnosis

Age-Related Testosterone Decline

  • Testosterone declines approximately 1–2% per year after age 30 — a normal physiological process distinct from pathological hypogonadism
  • At age 70, average testosterone is approximately 35–40% lower than peak levels at age 20–30
  • Late-onset hypogonadism (LOH): The clinical syndrome resulting from age-related testosterone decline; characterized by sexual dysfunction, reduced energy, mood changes, and body composition shifts
  • Accelerating factors: Obesity (adipose tissue aromatizes testosterone to estrogen), chronic illness, sleep apnea (reduces nocturnal testosterone surge), chronic stress (cortisol suppresses HPG axis), alcohol excess, opioid use, iron overload
  • Lifestyle contribution: A cross-sectional study of 1,667 men found that maintaining healthy weight, regular exercise, non-smoking, and adequate sleep preserved testosterone levels 10–20 years beyond those with poor lifestyle habits

Diagnosis & Testing

Hypogonadism requires both biochemical confirmation AND symptoms:

Symptoms of low testosterone:

  • Reduced libido and sexual desire (most specific symptom)
  • Erectile dysfunction
  • Reduced spontaneous morning erections
  • Fatigue, reduced energy and motivation
  • Decreased muscle mass and strength despite training
  • Increased body fat, particularly visceral/abdominal
  • Low mood, irritability, depression
  • Poor concentration and cognitive function
  • Reduced bone density (osteopenia/osteoporosis in severe/prolonged cases)

Recommended testing:

  • Total testosterone (morning sample, 8–10am when levels peak; fasting preferred)
  • Two confirmatory measurements on separate days if initially low
  • LH and FSH (to distinguish primary from secondary hypogonadism)
  • SHBG and calculated free testosterone
  • Prolactin, thyroid function, iron studies
  • Threshold for treatment: Generally total T below 300 ng/dL (10.4 nmol/L) with consistent symptoms; some guidelines use 264 ng/dL

Testosterone Replacement Therapy: Evidence

Benefits with strong evidence:

  • Sexual function: Libido and erectile function improve significantly in hypogonadal men; the most consistently documented TRT benefit across RCTs
  • Body composition: Reduces fat mass and increases lean muscle mass; particularly visceral fat reduction; requires resistance training for maximal effect
  • Bone density: Prevents and partially reverses osteoporosis in hypogonadal men
  • Mood and energy: Improvements in fatigue, motivation, and depression symptoms in men with confirmed hypogonadism
  • Anemia: Stimulates erythropoiesis; corrects anemia of hypogonadism

TRAVERSE Trial (2023) — cardiovascular safety resolved:

  • Landmark RCT (n=5,246, average age 63.5, cardiovascular risk or disease); testosterone gel vs placebo over 33 months
  • TRT did NOT increase major adverse cardiovascular events (MACE) — non-inferior to placebo
  • Notable finding: slightly increased atrial fibrillation (3.5% vs 2.4%), acute kidney injury, and pulmonary embolism — require monitoring
  • This trial has substantially changed prescribing confidence for appropriately selected men

Delivery methods:

  • Gels (AndroGel, Testim): Daily application; physiological levels; transfer risk to partners/children
  • Injections (testosterone cypionate/enanthate): Every 1–2 weeks; peak-trough fluctuations; more cost-effective
  • Pellets: Subcutaneous implant every 3–6 months; stable levels; minor surgical procedure
  • Patches: Daily application; skin irritation common

Monitoring and risks:

  • Hematocrit (polycythemia risk — most common side effect); PSA; testosterone levels; bone density
  • Suppresses endogenous testosterone production and sperm — fertility impact; clomiphene or HCG may preserve fertility
  • Prostate: TRT does not cause prostate cancer, but is contraindicated in active or high-risk prostate cancer

Lifestyle Optimization of Testosterone

  • Sleep: The single most impactful modifiable testosterone factor; 70% of daily testosterone is secreted during sleep; one week of sleep restriction (5h/night) reduces daytime testosterone by 10–15% in young men; optimizing sleep to 7–9h is foundational
  • Resistance training: Acutely raises testosterone; chronic resistance training programs increase baseline testosterone; compound movements (squats, deadlifts) produce the largest hormonal response; 3–4 sessions/week
  • Weight loss: Obesity is a major testosterone suppressant via aromatase-mediated conversion; each unit BMI reduction raises testosterone ~10 ng/dL; 10% weight loss can raise testosterone by 100–200 ng/dL in obese men
  • Stress reduction: Cortisol directly suppresses LH and testosterone production; chronic HPA axis activation is a significant contributor to low testosterone in high-stress individuals; evidence-based stress management is a genuine testosterone optimization strategy
  • Alcohol reduction: Chronic alcohol suppresses LH and directly damages Leydig cells; even moderate alcohol (2+ drinks/day consistently) reduces testosterone; cessation produces measurable increases within weeks
  • Vitamin D optimization: VDRs are present on Leydig cells; Vitamin D deficiency is associated with lower testosterone; RCT showed 3,332 IU/day Vitamin D raised testosterone by 20% vs placebo in deficient men

Evidence-Based Supplements

  • Zinc: Essential cofactor for testosterone synthesis; deficiency directly suppresses LH and testosterone; correction of deficiency restores levels; 25–40mg/day zinc (test levels first); common in athletes and plant-based dieters
  • Vitamin D: As above — 2,000–4,000 IU/day for deficient men; measure and target 40–60 ng/mL before titrating dose
  • Ashwagandha (KSM-66): Two RCTs specifically in hypogonadal or subfertile men show 10–17% testosterone increase, improved sperm quality, and reduced cortisol; mechanism via HPA axis normalization and direct Leydig cell stimulation; 300–600mg/day
  • Magnesium: Involved in testosterone synthesis and SHBG binding; deficiency associated with lower free testosterone; supplementation in deficient men raises free testosterone; 200–400mg/day
  • Fenugreek: Multiple RCTs show modest testosterone and free testosterone increases (12–17%); inhibits 5-alpha-reductase and aromatase; 500mg/day standardized extract
  • D-Aspartic Acid: Stimulates LH and testosterone release in some trials; inconsistent results — works better in hypogonadal than eugonadal men; 3g/day
  • Avoid: Many testosterone booster supplements contain proprietary blends with inadequate evidence; soy isoflavones at very high doses may slightly reduce testosterone (evidence weak); DHEA supplementation raises testosterone modestly in older men

Frequently Asked Questions

The most specific symptoms are reduced libido, decreased sexual desire, and loss of spontaneous morning erections. Other symptoms include fatigue, reduced motivation, decreased muscle mass and strength despite training, increased abdominal fat, low mood or depression, poor concentration, and reduced bone density in prolonged cases. Symptoms must be confirmed with blood testing — multiple conditions mimic low testosterone.

The 2023 TRAVERSE trial (n=5,246) provided strong evidence that TRT does not increase major cardiovascular events in men with pre-existing cardiovascular disease or risk factors. TRT is generally safe for appropriately selected men with confirmed hypogonadism. Risks include polycythemia (elevated red blood cell count), fertility suppression, and slightly increased atrial fibrillation and VTE risk in some populations. Regular monitoring (hematocrit, PSA, testosterone levels) is standard.

The most evidence-supported natural approaches are: optimizing sleep to 7–9 hours (70% of testosterone is secreted during sleep); resistance training 3–4x per week with compound movements; weight loss if overweight (each BMI unit lost raises testosterone ~10 ng/dL); stress and cortisol reduction; alcohol reduction; and correcting nutritional deficiencies (zinc, vitamin D, magnesium). These interventions can raise testosterone by 100–300 ng/dL — meaningful for men in the borderline-low range.

Most guidelines suggest treatment consideration when total testosterone is consistently below 300 ng/dL (10.4 nmol/L) with consistent symptoms, confirmed on two separate morning measurements. Some men with levels in the 300–400 ng/dL range with significant symptoms and low free testosterone may also be candidates. The decision requires a comprehensive clinical assessment — numbers alone do not determine treatment; symptoms, context, and goals matter.

No — current evidence does not support that TRT causes prostate cancer. The saturation model suggests prostate cells are already maximally stimulated at normal testosterone levels; supraphysiological levels (abuse doses) carry more concern. TRT is contraindicated in men with active or untreated prostate cancer, but does not appear to increase prostate cancer incidence in appropriately screened men. PSA monitoring during TRT is standard practice.

Research Summary

Male testosterone decline is normal with aging, but hypogonadism with symptoms is a clinically significant and treatable condition. The TRAVERSE trial has substantially clarified TRT cardiovascular safety.

  • Evidence strength: Strong (4/5)
  • Natural decline: 1–2% per year after age 30
  • Diagnosis threshold: Total T below 300 ng/dL + consistent symptoms
  • TRT: Strong evidence for libido, body composition, bone, and mood
  • TRAVERSE 2023: TRT does not increase MACE in high cardiovascular risk men
  • Lifestyle: Sleep, resistance training, and weight loss most evidence-supported
⚠️ Medical Disclaimer: This content is for informational purposes only and is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before making health decisions.

References

All studies cited are peer-reviewed. DOI and PubMed links open in a new tab.

  1. 1. Lincoff AM, Bhasin S, Flevaris P, et al. (TRAVERSE Investigators) (2023). Cardiovascular Safety of Testosterone-Replacement Therapy. New England Journal of Medicine, 389(2), 107–117. doi:10.1056/NEJMoa2215025 PMID:37326322
  2. 2. Bhasin S, Brito JP, Cunningham GR, et al. (2018). Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism, 103(5), 1715–1744. doi:10.1210/jc.2018-00229 PMID:29562364
  3. 3. Snyder PJ, Bhasin S, Cunningham GR, et al. (2016). Effects of Testosterone Treatment in Older Men. New England Journal of Medicine, 374(7), 611–624. doi:10.1056/NEJMoa1506119 PMID:26886521
  4. 4. Pilz S, Frisch S, Koertke H, et al. (2011). Effect of vitamin D supplementation on testosterone levels in men. Hormone and Metabolic Research, 43(3), 223–225. doi:10.1055/s-0030-1269854 PMID:21154195
  5. 5. Leproult R, Van Cauter E (2011). Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA, 305(21), 2173–2174. doi:10.1001/jama.2011.710 PMID:21632481
  6. 6. Wankhede S, Langade D, Joshi K, Sinha SR, Bhattacharyya S (2015). Examining the effect of Withania somnifera supplementation on muscle strength and recovery: a randomized controlled trial. Journal of the International Society of Sports Nutrition, 12, 43. doi:10.1186/s12970-015-0104-9 PMID:26609282
  7. 7. Corona G, Rastrelli G, Morgentaler A, Sforza A, Mannucci E, Maggi M (2017). Meta-analysis of results of testosterone therapy on sexual function based on international index of erectile function scores. European Urology, 72(6), 1000–1011. doi:10.1016/j.eururo.2017.03.032 PMID:28396078
  8. 8. Vingren JL, Kraemer WJ, Ratamess NA, Anderson JM, Volek JS, Maresh CM (2010). Testosterone physiology in resistance exercise and training: the up-stream regulatory elements. Sports Medicine, 40(12), 1037–1053. doi:10.2165/11536910-000000000-00000 PMID:21058750