What is the likely diagnosis and recommended workup for an elevated parathyroid hormone level of 91 pg/mL with normal serum calcium and normal vitamin D?

Normocalcemic Primary Hyperparathyroidism (NCHPTH)

This presentation of PTH 91 pg/mL with normal calcium (9.0 mg/dL) and normal vitamin D represents normocalcemic primary hyperparathyroidism (NCHPTH) until proven otherwise, but you must first systematically exclude all causes of secondary hyperparathyroidism before accepting this diagnosis.

Diagnostic Approach

The 2025 French expert consensus provides a clear algorithmic framework for this exact scenario 1. When PTH is elevated with normal calcium, you must:

Step 1: Rule Out Secondary Causes (Critical First Step)

Before diagnosing NCHPTH, you must exclude and correct all potential causes of secondary hyperparathyroidism 1:

• Verify vitamin D status is truly adequate: Confirm 25(OH)D has been maintained >30 ng/mL for >6 months, not just a single measurement 1
• Assess calcium intake: Low dietary calcium can cause secondary hyperparathyroidism even with normal vitamin D and renal function 2. A calcium challenge test (600 mg twice daily for 2-3 weeks) can confirm this—if PTH normalizes, the diagnosis was insufficient calcium intake 2
• Check renal function: Calculate eGFR to exclude chronic kidney disease
• Measure 24-hour urinary calcium: Profoundly low urinary calcium (<50-100 mg/24h) suggests chronic negative calcium balance 3
• Calculate fractional excretion of calcium (FeCa): If FeCa ≤2%, consider familial hypocalciuric hypercalcemia (FHH) 1

Step 2: Confirm NCHPTH Diagnosis

Once secondary causes are excluded, an oral calcium loading test can help clarify the mechanism (Grade A+ recommendation) 1. This test distinguishes true NCHPTH from residual secondary causes.

Step 3: Assess Target Organ Damage

NCHPTH is not merely a biochemical phenomenon—72% of patients have fragility fractures 4. The 2025 guidelines mandate systematic assessment 1:

• Bone evaluation (Grade A++):

  • DXA scan of spine, femur, and radius
  • Consider vertebral fracture assessment or spine X-rays for asymptomatic fractures

• Renal evaluation (Grade A++):

  • eGFR
  • 24-hour urine calcium and fasting calcium/creatinine ratio
  • Renal imaging: Low-dose CT scan is more sensitive than ultrasound for detecting nephrolithiasis/nephrocalcinosis 1

Clinical Context

Your PTH of 91 pg/mL is moderately elevated. NCHPTH affects approximately 3% of patients referred for osteoporosis evaluation 4, with mean PTH levels around 130 pg/mL in affected patients. This condition is:

• More common in postmenopausal women (75% female) 1
• Frequently associated with skeletal pathology (osteoporosis, fractures) 4
• Underdiagnosed when relying on calcium alone 4

Common Pitfalls to Avoid

1. Don't assume normal vitamin D on one measurement: Verify sustained adequacy >6 months 1
2. Don't skip the calcium intake assessment: This reversible cause is often missed 2, 3
3. Don't ignore low urinary calcium: This is a key diagnostic clue for negative calcium balance 3
4. Don't forget FeCa calculation: Essential to exclude FHH when considering NCHPTH 1

Specialist Referral Indications

• Nephrologist: If eGFR <60 mL/min/1.73m² or kidney stones present 1
• Urologist: If symptomatic stones, single kidney, or stones at risk of migration 1
• Endocrinologist: For definitive NCHPTH diagnosis and surgical candidacy evaluation

The key message: Do not diagnose NCHPTH prematurely. Complete the systematic exclusion of secondary causes first, particularly calcium intake assessment and sustained vitamin D adequacy, as these are readily treatable conditions that mimic NCHPTH 1, 2, 3.