What could cause one testicle to be smaller than the other and how can it be treated?

Small Testicles: Causes and Treatment

If you have 12ml testicles bilaterally, this represents testicular hypoplasia (small testes), and the most important step is determining the underlying cause through hormonal evaluation and assessment for conditions like cryptorchidism history, Klinefelter syndrome, varicocele, or hypogonadism, as testicular volume directly correlates with fertility potential and testosterone production. 1

Understanding Normal Testicular Size

• Normal adult testicular volume ranges from 15-25ml, with volumes measured using a Prader orchidometer or ultrasound 1
• Testicular volume of 12ml bilaterally is below the normal adult range and indicates testicular hypoplasia 1
• Testicular size directly correlates with spermatogenic function and fertility potential 1

Common Causes of Small Testicles

History of Cryptorchidism (Undescended Testicles)

• Men with a history of bilateral cryptorchidism have significantly reduced testicular volume and fertility, with paternity rates of only 35-53% compared to normal men 1
• Even men with unilateral cryptorchidism may have structural abnormalities in the contralateral descended testis, including smaller volume and reduced fertility markers 1
• Cryptorchidism increases risk of hypogonadism (low testosterone) and testicular cancer risk 3.6-7.4 times higher than the general population 1

Hypogonadism

• Small testes are a hallmark of primary hypogonadism, where the testes fail to produce adequate testosterone and sperm 1
• Hypogonadism can be congenital (present from birth, as in Prader-Willi syndrome or Klinefelter syndrome) or acquired 1
• Both males and females with certain genetic conditions are affected, though external manifestations may not be obvious 1

Varicocele

• Varicocele (enlarged veins in the scrotum) can cause progressive testicular atrophy and reduced volume over time 1
• Scrotal ultrasound can detect varicocele and assess testicular structure 1

Testicular Dysgenesis

• Non-homogeneous testicular architecture and microcalcifications on ultrasound suggest testicular dysgenesis, which is associated with impaired spermatogenesis and smaller testicular volume 1
• Men with testicular microcalcifications have an 18-fold higher prevalence of testicular cancer 1

Essential Diagnostic Workup

Physical Examination

• Palpation of both testes to assess volume, consistency, and position 1
• Examination for varicocele (enlarged veins that feel like "bag of worms") 1
• Assessment for signs of hypogonadism (reduced body hair, gynecomastia) 1

Hormonal Testing

• Serum testosterone (total and free) to assess Leydig cell function 1
• Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) to differentiate primary from secondary hypogonadism 1
• If FSH and LH are elevated with low testosterone, this indicates primary testicular failure 1

Scrotal Ultrasound

• Testicular volume measurement is more accurate than orchidometer in cases of small testes 1
• Assessment of testicular architecture for signs of dysgenesis (non-homogeneous pattern, microcalcifications) 1
• Detection of varicocele or other structural abnormalities 1
• Important caveat: Infertile men have higher risk of testicular cancer, and ultrasound may detect incidental masses requiring surveillance 1

Semen Analysis

• Essential for assessing fertility potential, as testicular volume correlates with sperm production 1
• Men with testicular volume <12ml often have oligozoospermia or azoospermia 1

Genetic Testing (If Indicated)

• Karyotype analysis if Klinefelter syndrome suspected (small firm testes, tall stature, gynecomastia) 1
• Y chromosome microdeletion testing if sperm concentration <5 million/ml 1

Treatment Options

Testosterone Replacement Therapy

• If hypogonadism is confirmed (low testosterone with symptoms), testosterone replacement therapy improves quality of life, sexual function, bone density, and muscle mass 1
• Critical warning: Testosterone replacement suppresses sperm production and should NOT be used in men desiring fertility 1

Fertility Preservation

• Men with small testes and impaired fertility should be counseled about sperm banking before any treatment that may further compromise fertility 1
• Assisted reproductive technologies (IVF with ICSI) may be necessary for conception 1

Surgical Intervention for Varicocele

• Varicocelectomy may improve testicular volume and sperm parameters in men with clinically significant varicocele 1

Treatment of Underlying Cause

• If cryptorchidism is present in adulthood with a normal contralateral testis, orchidopexy may be offered to preserve androgen production, though fertility benefit is limited 1
• For bilateral undescended testes in adults with impaired function, orchidopexy may preserve hormone production 1

Surveillance for Testicular Cancer

• Men with small testes, history of cryptorchidism, or testicular microcalcifications require lifelong surveillance with testicular self-examination 1
• Any new solid testicular mass warrants immediate evaluation with ultrasound and tumor markers (AFP, β-HCG, LDH) 1

Critical Pitfalls to Avoid

• Do not start testosterone replacement without first assessing fertility desires, as it will suppress sperm production irreversibly in some cases 1
• Do not ignore small testes as "normal variation"—they indicate underlying pathology requiring evaluation 1
• Do not delay fertility assessment if the patient desires future children, as testicular function may decline further with age 1
• Do not skip cancer surveillance in high-risk patients (cryptorchidism history, microcalcifications, infertility) 1