What laboratory tests should be obtained to evaluate an elevated parathyroid hormone (PTH) level?

Laboratory Evaluation of Elevated PTH

When PTH is elevated, immediately measure serum calcium (corrected for albumin or ionized), 25-hydroxyvitamin D, serum phosphorus, and creatinine with eGFR to distinguish primary hyperparathyroidism from secondary causes. 1, 2

Essential Initial Laboratory Panel

The diagnostic workup must include these specific tests simultaneously:

• Serum calcium – Use ionized calcium (normal 4.65-5.28 mg/dL) for definitive assessment, or correct total calcium for albumin if albumin is abnormal 1
• Intact PTH – Measured in EDTA plasma (not serum) as PTH is most stable in EDTA plasma at 4°C 3, 1
• 25-hydroxyvitamin D – Target >20 ng/mL (>50 nmol/L) to exclude vitamin D deficiency, the most common reversible cause of secondary hyperparathyroidism 1, 2
• Serum phosphorus – Typically low-normal in primary hyperparathyroidism versus elevated in CKD-related secondary hyperparathyroidism 1, 2
• Serum creatinine and eGFR – PTH rises when GFR falls below 60 mL/min/1.73 m², making CKD a critical differential 3, 1, 2

Additional Diagnostic Tests Based on Clinical Context

If Primary Hyperparathyroidism is Suspected (Elevated Calcium + Elevated PTH):

• 24-hour urine calcium or spot urine calcium/creatinine ratio – To exclude familial hypocalciuric hypercalcemia and assess for hypercalciuria (>300 mg/24hr indicates surgical candidacy) 1
• Alkaline phosphatase – May increase predictive power for high-turnover bone disease, though data are limited 3
• Bone density scan (DEXA) – To assess for osteoporosis (T-score ≤-2.5), which is a surgical indication 1
• Renal ultrasonography – To detect nephrocalcinosis or kidney stones 1

If Secondary Hyperparathyroidism is Suspected (Normal/Low Calcium + Elevated PTH):

• 1,25-dihydroxyvitamin D – Measure alongside 25-hydroxyvitamin D; the relationship provides critical diagnostic information 3, 1
  • In vitamin D deficiency: both are low
  • In granulomatous disease (sarcoidosis): 25-OH vitamin D is low but 1,25-(OH)₂ vitamin D is elevated due to increased 1α-hydroxylase activity 1
  • In CKD: 1,25-(OH)₂ vitamin D is typically low 3

If Hypercalcemia with Suppressed PTH:

• PTHrP (parathyroid hormone-related peptide) – Elevated in humoral hypercalcemia of malignancy, associated with median survival of approximately 1 month 1
• 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D together – To distinguish vitamin D intoxication (markedly elevated 25-OH vitamin D) from granulomatous disease 1

Critical Measurement Considerations

PTH Assay Variability:

• PTH assays vary by up to 47% between different generations (second-generation "intact PTH" vs. third-generation "whole PTH"), so always use assay-specific reference values 3, 1
• Second-generation assays overestimate biologically active PTH by detecting C-terminal fragments 3
• Biological variation of PTH is substantial – approximately 20% in healthy individuals and up to 30% in hemodialysis patients, meaning differences must exceed 54% in healthy people to be clinically significant 3

Preanalytical Factors:

• Use EDTA plasma, not serum – PTH is most stable in EDTA plasma 3, 1
• Biotin supplements can interfere with PTH assays, causing under- or overestimation depending on assay design 3
• Sampling site matters – Central blood has higher PTH concentrations than peripheral blood 1
• PTH increases with age due to steady decline in GFR, particularly in patients over 60 years old 3, 1, 2
• PTH is race-dependent – higher in Black compared to White individuals 3
• PTH correlates with BMI 3

Monitoring Frequency by CKD Stage

For patients with chronic kidney disease (not on dialysis):

• CKD G3a-G3b (eGFR 30-59 mL/min/1.73 m²): Measure calcium, phosphorus, and PTH every 6-12 months 2
• CKD G4 (eGFR 15-29 mL/min/1.73 m²): Measure every 3-6 months 2
• CKD G5 (eGFR <15 mL/min/1.73 m²): Measure every 1-3 months 2

During active treatment of secondary hyperparathyroidism, check calcium and phosphorus monthly for the first 3 months, then every 3 months; measure PTH every 3 months for 6 months, then every 3-6 months 2

Common Pitfalls to Avoid

• Do not order parathyroid imaging before confirming biochemical diagnosis – Imaging is for surgical planning, not diagnosis 1
• Do not diagnose normocalcemic primary hyperparathyroidism without excluding all secondary causes – Vitamin D deficiency, inadequate calcium intake, and CKD must be ruled out first 1, 2, 4
• Do not use a single PTH measurement – Repeat after 3 months to account for 20% biological variability 2
• Do not supplement with vitamin D until hypercalcemia is resolved if calcium exceeds 10.2 mg/dL 1
• Do not prescribe calcitriol or active vitamin D analogs in primary hyperparathyroidism – They increase intestinal calcium absorption and worsen hypercalcemia 1