Causes of Hypouricemia
Hypouricemia results from either decreased uric acid production or increased renal excretion, with the most common causes being medications (particularly urate-lowering therapies), hereditary renal transport defects, and severe underlying diseases affecting liver or kidney function.
Definition
Hypouricemia is defined as serum uric acid concentration <2.0 mg/dL (119 μmol/L) 1, 2, 3.
Primary Classification
The etiologies divide into two main pathophysiologic categories:
1. Decreased Uric Acid Production (Underproduction)
Hereditary xanthinuria: Autosomal recessive disorder caused by mutations in xanthine oxidase, leading to minimal urate production with elevated xanthine levels 2, 4
- Results in very low serum urate levels
- Associated with xanthine stones, hematuria, and potential chronic kidney disease
- Serum uric acid may be 0 mg/dL in severe cases 4
Alkaptonuria: Rare metabolic disorder 4
2. Increased Renal Excretion (Overexcretion)
Hereditary renal hypouricemia (RHUC): Genetic defects in renal urate transporters 2, 3, 5
Fanconi syndrome: Proximal tubular dysfunction with impaired reabsorption 6
Nephropathic cystinosis: Most common cause in the overexcretion group among pediatric patients 4
Distal renal tubular acidosis 4
Secondary Causes
Medications (Account for approximately 50% of cases) 6, 7
Urate-lowering therapies:
- Xanthine oxidase inhibitors: allopurinol, febuxostat 1, 8
- Uricosuric agents: probenecid, benzbromarone 1
- URAT1 inhibitors: lesinurad 1
- Recombinant uricase: pegloticase 1
Other medications:
- Trimethoprim-sulfamethoxazole (TMP-SMX), especially high-dose 1, 8
- Angiotensin II receptor blockers: losartan 1
- Fenofibrate 1
- High-dose salicylates 1
- Some NSAIDs 1
Underlying Diseases 6
- Severe hepatocellular disease 6
- Neoplasia 6
- Diabetes mellitus 6
- AIDS 6
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH) 6
Nutritional and Metabolic Factors 8
- Amino acids in parenteral nutrition 8
- Emaciation/severe malnutrition 8
- Hyponatremia (low serum sodium levels) 8
Clinical Pitfalls
Key diagnostic approach: Measure uric acid clearance and fractional excretion of uric acid to distinguish between underproduction (normal/reduced clearance) versus overexcretion (increased clearance) 6.
Important warning: While hypouricemia is often asymptomatic, it can predispose to serious complications including:
- Exercise-induced acute kidney injury (particularly in RHUC) 3, 5
- Nephrolithiasis (xanthine or uric acid stones) 6, 2
- Acute renal failure with oxidant stress 6
Diagnostic lag: Isolated hypouricemia without metabolic acidosis tends to have longer diagnostic delays compared to cases with accompanying metabolic abnormalities 4. A serum uric acid level of 0 mg/dL strongly suggests underproduction as the etiology 4.
Multiple myeloma context: Hypouricemia can occur in multiple myeloma patients with Fanconi syndrome due to light chain-induced proximal tubular dysfunction 9.