Effect of Intratesticular Testosterone on Testicle Size
Intratesticular testosterone administration can cause testicular atrophy, while suppression of intratesticular testosterone through exogenous testosterone administration leads to decreased testicular size. 1, 2
Mechanisms of Testosterone's Effect on Testicular Size
- Intratesticular testosterone (ITT) is essential for normal spermatogenesis and testicular function, with levels approximately 40 times higher than serum testosterone in healthy males 3
- Direct intratesticular testosterone administration can paradoxically cause testicular atrophy, as demonstrated in animal studies where intratesticular injections led to decreased testicular weight and volume 1
- Exogenous testosterone administration (via injections, gels, or patches) suppresses the hypothalamic-pituitary-gonadal axis through negative feedback, reducing gonadotropin secretion and consequently decreasing intratesticular testosterone production 4
- This suppression of natural testosterone production results in testicular volume reduction, with studies showing 16-19% decrease in testicular volume after 4 months of testosterone enanthate treatment 2
Clinical Evidence and Observations
- In men receiving testosterone replacement therapy, testicular size and consistency often diminish due to down-regulation of gonadotropins 5
- The degree of testicular volume reduction correlates directly with the decrease in sperm count during exogenous testosterone administration 2
- Studies show that when exogenous testosterone is discontinued, testicular volume typically returns to normal size 2
- Even very low intratesticular testosterone levels (as low as 2% of normal) can maintain some degree of spermatogenesis, though not at normal levels 6
Clinical Implications
- Men should be advised that fertility will be greatly compromised during testosterone replacement therapy due to the suppression of natural testosterone production and subsequent reduction in testicular size 5
- Testicular size prior to treatment correlates with response to human chorionic gonadotropin (hCG) therapy, which is used to restore testicular function after anabolic steroid use 4
- For men concerned about fertility, exogenous testosterone therapy should be avoided as it can further suppress the hypothalamic-pituitary-gonadal axis 4
- Alternative treatments like hCG can directly stimulate testicular Leydig cells to produce testosterone without causing the same degree of testicular atrophy 4
Monitoring Considerations
- Testicular size should be monitored in patients receiving testosterone replacement therapy as a clinical indicator of the degree of gonadotropin suppression 5
- Currently, the only direct method to assess intratesticular testosterone levels is through testicular biopsy, which is invasive and can lead to complications 7
- Research is ongoing to identify serum biomarkers that could serve as surrogate indicators of intratesticular testosterone levels 7
Understanding the relationship between intratesticular testosterone and testicular size has important implications for male contraception research, testosterone replacement therapy, and fertility preservation strategies.