What are kidney stones due to primary hyperparathyroidism (PHPT) composed of?

Composition of Kidney Stones in Primary Hyperparathyroidism

Kidney stones in primary hyperparathyroidism (PHPT) are predominantly composed of calcium phosphate, often mixed with calcium oxalate. While most idiopathic kidney stones are primarily calcium oxalate, PHPT shifts the balance toward calcium phosphate composition due to the metabolic changes associated with the condition.

Stone Composition Characteristics

Primary Components

• Calcium phosphate: The predominant component in PHPT stones
  • Often present as carbonate apatite or brushite
  • Higher proportion compared to idiopathic stone formers
• Calcium oxalate: Frequently present as a secondary component
  • May be mixed with calcium phosphate in varying proportions
  • Can appear as monohydrate or dihydrate forms

Factors Influencing Stone Composition in PHPT

1. Elevated urinary calcium excretion

   • Hypercalciuria is a direct result of elevated PTH levels
   • Increases calcium availability for stone formation 1

2. Urinary pH alterations

   • Higher urinary pH in many PHPT patients
   • Alkaline urine favors calcium phosphate crystallization over calcium oxalate 2

3. Supersaturation levels

   • Increased calcium phosphate supersaturation
   • Elevated calcium oxalate supersaturation 3, 4

Clinical Implications

Diagnostic Value

• Stone composition analysis can provide valuable diagnostic clues
• The American Urological Association recommends obtaining stone analysis at least once when available 5
• Predominance of calcium phosphate may suggest underlying PHPT, especially when combined with other clinical features

Treatment Considerations

• Successful parathyroidectomy significantly decreases:

  • Serum calcium (from 10.8 to 9.3 mg/dl)
  • Urinary calcium excretion (from 319 to 156 mg per day)
  • Calcium oxalate supersaturation (from 8.6 to 5.7)
  • Calcium phosphate supersaturation (from 1.6 to 0.9) 3, 4

• Despite successful surgery, approximately 47% of patients may have persistent hypercalciuria and continued stone risk 4

Important Caveats

• Not all PHPT patients form stones: Despite similar levels of hypercalcemia and hypercalciuria, only a subset of PHPT patients develop kidney stones, suggesting other contributing factors 3

• Post-surgical monitoring: Even after successful parathyroidectomy, patients should be monitored for stone recurrence as underlying idiopathic hypercalciuria may be unmasked 4

• Distinguishing features: While calcium phosphate predominance suggests PHPT, stone analysis alone is insufficient for diagnosis and must be correlated with biochemical findings (serum calcium, PTH levels) 5, 1

Monitoring Recommendations

• 24-hour urine collections should be performed before and after parathyroidectomy to assess:

  • Urinary calcium excretion
  • Stone risk factors
  • Supersaturation indices for calcium oxalate and calcium phosphate 5, 1

• Follow-up imaging to monitor for new stone formation or growth of existing stones 5