What is the most likely diagnosis and recommended next steps for a patient with elevated parathyroid hormone (PTH) of 104 pg/mL, normal total calcium (9.2 mg/dL), normal ionized calcium (5.2 mg/dL), and normal 24‑hour urinary calcium excretion?

Normocalcemic Primary Hyperparathyroidism

This patient has normocalcemic primary hyperparathyroidism (PHPT), and you must first rigorously exclude all causes of secondary hyperparathyroidism before accepting this diagnosis.

Most Likely Diagnosis

The biochemical pattern of elevated PTH (104 pg/mL) with normal total calcium (9.2 mg/dL), normal ionized calcium (5.2 mg/dL), and normal 24-hour urinary calcium represents an atypical form of primary hyperparathyroidism characterized by persistently elevated PTH in the absence of hypercalcemia 1.

However, this is a diagnosis of exclusion—you cannot make this diagnosis until you have systematically ruled out secondary causes.

Critical Next Steps: Rule Out Secondary Hyperparathyroidism

Before accepting normocalcemic PHPT, you must exclude the following causes of secondary hyperparathyroidism 1:

1. Vitamin D Status (Most Important)

• Measure 25-hydroxyvitamin D levels
• The patient must have maintained normal vitamin D levels for >6 months before diagnosing normocalcemic PHPT 1
• Vitamin D deficiency is the most common cause of inappropriately elevated PTH with normal calcium

2. Renal Function

• Calculate estimated glomerular filtration rate (eGFR)
• Even mild chronic kidney disease can cause secondary hyperparathyroidism
• Rule out early renal insufficiency

3. Medications

• Review for thiazide diuretics, lithium, or other medications affecting calcium-PTH axis

4. Malabsorption Syndromes

• Screen for celiac disease, inflammatory bowel disease, or prior bariatric surgery
• These can cause secondary hyperparathyroidism despite normal calcium

5. Hypercalciuria

• Calculate fractional excretion of calcium (FeCa)
• FeCa >2% supports PHPT; FeCa ≤2% suggests familial hypocalciuric hypercalcemia (FHH) 1
• Formula: FeCa = (urinary calcium × plasma creatinine) / (urinary creatinine × plasma calcium)

Confirming the Diagnosis

Once secondary causes are excluded, confirm normocalcemic PHPT by:

• Repeat measurements: Document elevated PTH on at least 2 occasions over 3-6 months with persistently normal total AND ionized calcium 2
• Oral calcium loading test: Can help clarify the mechanism of elevated PTH (Grade A+) 1
• Consider genetic testing: Up to 10% of PHPT cases are hereditary 3

Assessment of Target Organ Damage

If normocalcemic PHPT is confirmed, systematically assess for complications 1:

Bone Assessment

• Bone densitometry (DXA): Measure spine, femoral neck, and distal radius
• Vertebral imaging: Screen for asymptomatic vertebral fractures
• Look for T-score ≤-2.5 SD at any site

Renal Assessment

• Renal imaging: Low-dose CT scan (more sensitive than ultrasound) to detect nephrolithiasis or nephrocalcinosis 1
• Screen for history of kidney stones
• Assess for eGFR <60 mL/min/1.73 m²

Treatment Considerations

Surgery is indicated if any of the following criteria are present 4:

• Bone: T-score ≤-2.5 SD or low-energy fracture
• Renal: Kidney stones, nephrocalcinosis, or eGFR <60 mL/min/1.73 m²
• Hypercalciuria: 24-hour urine calcium >250 mg/day (female) or >300 mg/day (male)
• Age: <50 years (formally), but consider up to <70 based on life expectancy
• Calcium: >10 mg/L above upper limit of normal (though this patient is normocalcemic)

Important Caveats

Normocalcemic PHPT represents approximately 30% of patients with PHPT who have total calcium <2.6 mmol/L (<10.4 mg/dL) 4. The natural history remains incompletely understood 2, 5, and not all patients progress to hypercalcemia 5.

Exercise caution before recommending surgery unless clear target organ damage is present 2. The risk-benefit ratio should be carefully assessed, particularly in asymptomatic patients without bone or renal complications.

Common pitfall: Failing to adequately exclude vitamin D deficiency and other secondary causes leads to misdiagnosis. The PTH elevation must be "inappropriate" relative to calcium levels after maintaining normal vitamin D for >6 months 1.

If diagnostic uncertainty persists after excluding secondary causes, the oral calcium loading test can provide additional diagnostic clarity by demonstrating autonomous parathyroid function 1.