Laboratory Tests for Patients with Hypercalcemia and Elevated iPTH
For patients with hypercalcemia and elevated intact parathyroid hormone (iPTH), a comprehensive laboratory evaluation should include serum blood urea nitrogen, creatinine, vitamin D concentrations (25-hydroxyvitamin D and 1,25-dihydroxyvitamin D), and urine calcium/creatinine ratio to assess for hypercalciuria. 1
Essential Laboratory Tests
Initial Evaluation
- Serum blood urea nitrogen (BUN) and creatinine to assess kidney function, as hypercalcemia can lead to kidney damage 1
- Vitamin D concentrations, including both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, to exclude vitamin D deficiency as a secondary cause of elevated PTH 1
- Urine calcium/creatinine ratio in a random spot urine to evaluate for hypercalciuria 1
- Serum albumin to allow for accurate interpretation of total calcium levels 2
- Serum phosphorus, as hypophosphatemia often accompanies primary hyperparathyroidism 3
Additional Tests Based on Initial Findings
- Renal ultrasonography if hypercalciuria is present to evaluate for nephrocalcinosis 1
- Serum ionized calcium measurement for more accurate assessment of calcium status, especially in patients with abnormal albumin levels 2
Monitoring Parameters
- Serial assessments of phosphate, calcium, and PTH levels should be considered together for ongoing management 1
- For patients with confirmed primary hyperparathyroidism, calcium levels should be monitored approximately every 2 months 4
- For patients with secondary hyperparathyroidism on treatment, monitor serum calcium and phosphorus at least monthly for the first 3 months after initiating therapy, then every 3 months thereafter 1
Special Considerations
Distinguishing Between Types of Hyperparathyroidism
- The combination of hypercalcemia with elevated or inappropriately normal iPTH strongly suggests primary hyperparathyroidism 5
- Even iPTH values in the normal range (but not suppressed) in the presence of hypercalcemia are consistent with primary hyperparathyroidism 6
- Consider familial hypocalciuric hypercalcemia (FHH) in patients with family history of hypercalcemia - typically characterized by low urinary calcium excretion (FeCa < 2%), though atypical presentations with normal urinary calcium can occur 3
Pitfalls to Avoid
- Do not rely solely on total calcium levels; albumin-adjusted or ionized calcium measurements provide more accurate assessment 2
- Avoid overlooking vitamin D deficiency, which can coexist with and exacerbate hyperparathyroidism 1
- Be aware that PTH assay differences can affect interpretation - use assay-specific reference ranges 1
- Remember that biotin supplements can interfere with some PTH assays, potentially causing falsely elevated or decreased results 1
When to Consider Referral
- Referral to a nephrologist and/or endocrinologist should be considered for management of persistent hypercalcemia, hypercalciuria, or nephrocalcinosis 1
- Surgical referral may be appropriate for patients with primary hyperparathyroidism who meet criteria for parathyroidectomy 6
By following this structured laboratory assessment approach, clinicians can effectively evaluate the cause and severity of hypercalcemia with elevated iPTH, monitor treatment response, and identify potential complications that may affect patient morbidity and mortality.