What labs and imaging should be ordered to evaluate suspected hyperparathyroidism?

Testing for Hyperparathyroidism

Diagnose hyperparathyroidism biochemically with serum calcium, parathyroid hormone (PTH), phosphate, and vitamin D levels—imaging is NOT used to confirm or exclude the diagnosis, only to localize abnormal glands before surgery. 1

Laboratory Testing

Essential Initial Labs

• Serum calcium (total and ionized): Elevated in primary hyperparathyroidism (PHPT), low-normal or low in secondary hyperparathyroidism (SHPT) 1, 2
• Parathyroid hormone (PTH): Elevated or inappropriately normal in PHPT; markedly elevated in SHPT 1, 3
• Serum phosphate: Typically low in PHPT (mean 0.70 ± 0.19 mmol/L); elevated in SHPT due to chronic kidney disease 4
• 25-hydroxyvitamin D: Essential to differentiate SHPT from PHPT, as vitamin D deficiency causes secondary hyperparathyroidism 1

Additional Useful Labs

• Serum chloride: Used in diagnostic ratios 4
• Creatinine/eGFR: Assess renal function to identify chronic kidney disease as cause of SHPT 1
• 24-hour urine calcium: Helps exclude familial hypocalciuric hypercalcemia 3

Diagnostic Biochemical Ratios

These simple calculations improve diagnostic accuracy for PHPT 4:

• Ca/P ratio: Significantly elevated in PHPT (4.17 ± 1.21); sensitivity 77.6%, NPV 86.6% 4
• Ca × Cl/P ratio: Highest specificity (89.2%) and PPV (82.2%) for PHPT (448.5 ± 133.6) 4
• PF Index (Ca × PTH/P): Moderate accuracy (AUC 0.851) for PHPT 4

Important caveat: These ratios cannot reliably distinguish normocalcemic PHPT from vitamin D deficiency-related SHPT, which show no significant differences (p = 0.63-0.74) 4

Imaging Studies

Critical Principle

Imaging has NO utility in confirming or excluding the diagnosis of hyperparathyroidism—it is used solely for preoperative localization of abnormal parathyroid glands to facilitate targeted surgery. 1

When to Order Imaging

• Primary hyperparathyroidism: Only after biochemical diagnosis is established and surgery is planned 1
• Persistent/recurrent PHPT: Preoperative imaging is essential before reoperation due to higher surgical complexity 1
• Secondary/tertiary hyperparathyroidism: When medical management fails and surgery is considered, imaging identifies all hyperplastic glands including ectopic locations 1

First-Line Imaging Options

No universally accepted algorithm exists; selection depends on surgeon/radiologist preference, local expertise, and patient characteristics (suspected multiglandular disease, hereditary syndromes, concomitant thyroid disease). 1

The most commonly recommended first-line approach combines 5:

• Cervical ultrasound (by experienced parathyroid sonographer) PLUS
• 99mTc-sestamibi SPECT or SPECT/CT (tomographic imaging superior to planar scintigraphy) 5

This dual-modality approach maximizes accuracy through concordant localization 1, 5

Alternative/Advanced Imaging

• 4D-CT (CT neck without and with IV contrast): Uses 3-phase protocol (noncontrast, arterial, venous) to identify parathyroid adenomas based on perfusion characteristics; similar diagnostic performance to SPECT/CT but higher radiation dose 1, 5
• 4D-MRI: Alternative when first-line studies are negative or discordant 5
• 18F-fluorocholine PET or 11C-methionine PET: Reserved for negative/discordant conventional imaging 6, 5

Highest Diagnostic Performance

99mTc-sestamibi SPECT/CT and 4D-CT demonstrate the highest detection rates for parathyroid adenomas, but the optimal strategy balances diagnostic accuracy with radiation exposure, cost, and local availability 5

Clinical Context Matters

Primary vs Secondary Hyperparathyroidism

• PHPT: Hypercalcemia with elevated/inappropriately normal PTH 1, 2
• SHPT: Hypocalcemia/hyperphosphatemia with elevated PTH, most commonly from chronic kidney disease, malabsorption, or vitamin D deficiency 1
• Tertiary HPT: Hypercalcemia with elevated PTH in long-standing SHPT (often post-kidney transplant) 1

PTH Assay Considerations

Be aware that different PTH assay generations and lack of complete standardization can cause significant inter-laboratory variation in PTH values, potentially affecting clinical decisions. 1 Trend monitoring within the same laboratory is more reliable than comparing absolute values across different facilities 1.