What could be the cause of a 0.9cm decrease in testicle length, from 4cm to 3.1cm, over a 4-week period, as shown on two ultrasound scans (USS) of my testicles?

Testicular Measurement Variability on Ultrasound

The 0.9 cm difference between your two ultrasound measurements (4 cm vs 3.1 cm) is almost certainly due to measurement error rather than true testicular atrophy, as true biological change in testicular size over just 4 weeks is extremely unlikely in adults unless acute pathology is present. 1

Why Measurement Error is the Most Likely Explanation

Technical factors cause significant variability in testicular measurements between ultrasound examinations:

• Incorrect caliper placement is the most common source of error, particularly when measuring testicular width, which can lead to severely inaccurate volume calculations 1
• High-frequency probes (>10 MHz) should be used to maximize resolution and accurate caliper placement, and measurements should include three perpendicular dimensions (length, width, height) on axial slices 1
• The same sonographer should perform serial measurements when possible, or measurements should be remeasured on previous scans by the current operator to minimize inter-scan variability 1
• Even with experienced operators, interobserver variability exists, though studies show good to excellent agreement (ICC 0.82-0.98) when proper technique is used 2

Understanding Your Measurements

A 4 cm testicular length corresponds to a volume of approximately 15-18 ml, which is within normal range, while a 3.1 cm length would suggest significant atrophy (approximately 8-10 ml): 1

• Testicular volumes less than 12 ml are definitively considered atrophic and associated with significant pathology, including impaired spermatogenesis and increased risk of intratubular germ cell neoplasia 1
• The American College of Radiology suggests that if there is a size discrepancy between measurements greater than 2 ml or 20%, this warrants investigation to confirm accuracy 1

What You Should Do Next

Request a repeat scrotal ultrasound with explicit attention to proper measurement technique to confirm testicular volume: 1

• The Lambert formula (Length × Width × Height × 0.71) should be used for calculating testicular volume, as it provides the most accurate estimates compared to actual testicular volume 1
• Do not use the traditional ellipsoid formula (0.52 coefficient) for clinical decision-making, as it systematically underestimates volume and may lead to inappropriate classification of testicular atrophy 1
• Compare measurements to the contralateral testis, as size discrepancy greater than 2 ml or 20% warrants further evaluation to exclude pathology 1

When to Worry About True Atrophy

If repeat ultrasound confirms small testicular volume (<12 ml), obtain hormonal evaluation (FSH, LH, testosterone): 1

• If you have normal fertility, no history of cryptorchidism (undescended testicles), and normal secondary sexual characteristics, the likelihood of true severe testicular atrophy is low, further supporting measurement error 1
• Conditions associated with smaller testicular volumes include a history of cryptorchidism, which substantially increases cancer risk and mandates closer surveillance 1
• Age under 30-40 years with testicular volume <12 ml carries a ≥34% risk of intratubular germ cell neoplasia in the contralateral testis if testicular cancer is present 1

Common Pitfalls to Avoid

Technical measurement errors are far more common than rapid testicular atrophy:

• Minor technical problems in caliper placement during ultrasound may lead to incorrect testicular measurements, which can result in severely atrophic and inconsistent volume calculations 1
• True biological change in testicular size over 4 weeks is extremely unlikely in adults unless there is acute pathology such as testicular torsion, trauma, or acute orchitis 1, 3
• Single measurements can be misleading—serial measurements with standardized technique are essential for accurate assessment 1