What could cause a 50-year-old male's right kidney to be 50% larger than average?

Unilateral Kidney Enlargement in a 50-Year-Old Male

A 50-year-old male with a right kidney 50% larger than average most likely has either a renal mass (benign or malignant), polycystic kidney disease (particularly atypical unilateral ADPKD), or compensatory hypertrophy from contralateral kidney dysfunction.

Primary Diagnostic Considerations

Renal Mass (Most Critical to Rule Out)

High-quality, multiphase, cross-sectional abdominal imaging (CT or MRI) must be obtained immediately to characterize the mass, assess for enhancement, and clinically stage any potential malignancy 1. This is the most urgent consideration because:

• Renal cell carcinoma and other solid renal masses can cause significant kidney enlargement 1
• Over 50% of renal masses are now diagnosed incidentally, and early detection dramatically impacts morbidity and mortality 1
• The imaging should specifically assess tumor complexity, degree of contrast enhancement, and presence or absence of fat 1

If imaging reveals a solid or complex cystic mass, obtain comprehensive metabolic panel, complete blood count, urinalysis, and chest imaging for metastatic evaluation 1.

Atypical Unilateral ADPKD

The 2025 KDIGO guidelines specifically describe unilateral ADPKD as "diffuse cystic involvement of one kidney causing marked kidney enlargement with a normal contralateral kidney" 1. This presentation:

• Occurs in a distinct subset of ADPKD patients with asymmetric disease 1
• The contralateral kidney must have normal volume (<275 ml in men) with no or only 1-2 cysts 1
• Requires genetic testing if suspected, particularly in patients ≤46 years of age or with family history 1

Compensatory Hypertrophy

Kidney length strongly correlates with eGFR and body surface area 2, 3. Unilateral enlargement may indicate:

• Contralateral kidney dysfunction or atrophy requiring the enlarged kidney to compensate 3
• The contralateral kidney must be carefully evaluated for size, function, and structural abnormalities 1
• Cortical volume increases with higher GFR, and this relationship is independent of age 3

Essential Diagnostic Workup Algorithm

Step 1: Immediate Imaging

• Obtain multiphase contrast-enhanced CT or MRI of abdomen to characterize both kidneys 1
• Measure exact kidney volumes and assess for masses, cysts, or structural abnormalities 1

Step 2: Laboratory Evaluation

• Comprehensive metabolic panel (including creatinine/eGFR) 1
• Complete blood count 1
• Urinalysis with assessment for proteinuria and hematuria 1
• If mass suspected: chest imaging for metastatic evaluation 1

Step 3: Risk Stratification Based on Imaging

• If solid or complex cystic mass present: Urgent urology referral for renal mass biopsy consideration and treatment planning 1
• If multiple cysts with unilateral enlargement: Consider genetic testing for ADPKD and refer to nephrology 1
• If contralateral kidney is small/atrophic: Evaluate for chronic kidney disease and causes of unilateral kidney dysfunction 4

Critical Pitfalls to Avoid

Do not assume benign etiology based on age alone—renal cell carcinoma occurs across all adult age groups, and 50 years old is within the typical range 1.

Do not delay imaging—any unexplained kidney enlargement of this magnitude (50% above normal) requires immediate cross-sectional imaging to exclude malignancy 1.

Do not overlook the contralateral kidney—the finding of unilateral enlargement mandates careful evaluation of the opposite kidney, as its status fundamentally changes the differential diagnosis 1, 5.

Recognize that "normal" kidney size does not exclude chronic kidney disease, particularly in diabetic nephropathy or early-stage disease 4. However, 50% enlargement is distinctly abnormal and requires investigation.

Less Common but Important Considerations

• Vascular anomalies (twisted renal vessels) can alter kidney shape but rarely cause 50% size increase 6
• Hepatic pathology (cirrhosis with hepatatrophy) can alter kidney position but not typically size 5
• Large kidney length (>11.7 cm) is independently associated with higher cardiovascular mortality risk, irrespective of eGFR 2