Testosterone

Mechanism of Action

Testosterone exerts its effects by binding to the androgen receptor (AR), a nuclear receptor that acts as a ligand-activated transcription factor. Once bound, the testosterone-AR complex translocates to the nucleus and modulates gene expression, driving anabolic processes including muscle protein synthesis and erythropoiesis, while regulating feedback on the hypothalamic-pituitary-gonadal (HPG) axis.

Testosterone undergoes two key conversions in peripheral tissues:

• 5α-reductase converts testosterone to dihydrotestosterone (DHT), a more potent AR agonist responsible for prostate growth, scalp hair loss, and certain androgenic effects.
• Aromatase (CYP19A1) converts testosterone to estradiol (E2), essential for bone density, cardiovascular health, libido, and mood regulation in males.

Exogenous testosterone suppresses LH and FSH via negative feedback on the HPG axis, reducing endogenous testicular production. Co-administration of HCG or Gonadorelin is used in protocols to maintain testicular function and intratesticular testosterone.

Research Summary

Hypogonadism & TRT

Multiple large trials confirm testosterone replacement significantly improves serum T levels, sexual function, muscle mass, bone density, and mood in hypogonadal males. The TRAVERSE trial (n=5,204) found no increased risk of major adverse cardiovascular events (MACE) in men treated with testosterone gel, providing key safety data for long-term TRT.

Body Composition

Dose-response studies show supraphysiological testosterone doses produce graded increases in fat-free mass and strength. Even at physiological replacement doses (100 mg/week), testosterone improves lean mass and reduces adiposity in hypogonadal men.

Bone Mineral Density

Testosterone (via aromatization to estradiol) is critical for bone remodeling. TRT increases BMD at the lumbar spine and femoral neck in hypogonadal men, with effects mediated primarily through estradiol rather than testosterone directly.

Erythropoiesis

Testosterone stimulates erythropoietin production and directly acts on bone marrow precursors, increasing hematocrit and hemoglobin. Polycythemia (elevated hematocrit >54%) is the most common dose-dependent side effect requiring monitoring.

Research Protocols

TRT Protocol - Testosterone Cypionate

Dose Quick Reference

Common research doses: 50 mg 100 mg 125 mg 150 mg 200 mg

Draw Volume (200 mg/mL)

Monitoring Parameters

• Total testosterone: Target 500–900 ng/dL (trough level, day of injection)
• Hematocrit: Monitor every 3–6 months; hold if >54%
• Estradiol (E2): Target 20–40 pg/mL; aromatase inhibitors if elevated
• PSA: Baseline and periodic monitoring in males over 40
• LH/FSH: Will be suppressed; use HCG if fertility preservation needed

Storage & Handling

Testosterone oil solutions are highly stable at room temperature. Store sealed vials between 20-25°C, protected from light. Do not refrigerate. Cold temperatures can cause the oil to thicken, making it difficult to draw. Once opened, use within 28 days. Always inspect for particulate matter or discoloration before drawing.

Key notes:

• No reconstitution needed. Comes pre-mixed as an oil solution
• Use an 18-gauge drawing needle to overcome oil viscosity; switch to 23-25g for injection
• Warming the vial to body temperature (palm of hand for 30 seconds) reduces viscosity
• Never freeze or microwave the vial

References

1. Lincoff AM, et al. "Cardiovascular Safety of Testosterone-Replacement Therapy." N Engl J Med. 2023;389(2):107-117. (TRAVERSE trial)
2. Bhasin S, et al. "Testosterone Dose-Response Relationships in Healthy Young Men." Am J Physiol Endocrinol Metab. 2001;281(6):E1172-81.
3. Snyder PJ, et al. "Effects of Testosterone Treatment in Older Men." N Engl J Med. 2016;374(7):611-624.
4. Finkelstein JS, et al. "Gonadal Steroids and Body Composition, Strength, and Sexual Function in Men." N Engl J Med. 2013;369(11):1011-1022.
5. Khera M. "Male Hormonal Contraception." Fertil Steril. 2013;99(3):618-622.