Causes of Elevated Parathyroid Hormone (PTH)
Elevated PTH levels result from either primary hyperparathyroidism (autonomous parathyroid gland overproduction), secondary hyperparathyroidism (appropriate physiologic response to hypocalcemia or other stimuli), tertiary hyperparathyroidism (autonomous PTH secretion after chronic secondary hyperparathyroidism), or spurious laboratory elevation from assay interference.
Primary Hyperparathyroidism (PHPT)
Primary hyperparathyroidism is the most common pathologic cause of elevated PTH and presents with hypercalcemia and inappropriately normal or elevated PTH levels 1, 2.
Etiology of PHPT:
- Single parathyroid adenoma (most common cause - approximately 80-85% of cases) 2
- Multiglandular disease (parathyroid hyperplasia)
- Familial syndromes: Multiple Endocrine Neoplasia (MEN) types 1 and 2A 3
- Parathyroid carcinoma (rare) 3
Biochemical Pattern:
- Elevated or inappropriately normal PTH with hypercalcemia
- Hypophosphatemia
- Increased or normal urinary calcium excretion 1
Normocalcemic Variant:
A recognized variant exists where PTH is persistently elevated with consistently normal serum calcium, after excluding all causes of secondary hyperparathyroidism 2. This requires careful exclusion of vitamin D deficiency, renal insufficiency, and other secondary causes 1.
Secondary Hyperparathyroidism
Secondary hyperparathyroidism represents an appropriate physiologic response where PTH elevation occurs with normal or low calcium levels 4.
Major Causes:
Chronic Kidney Disease (most common cause):
Vitamin D Deficiency:
- Reduced intestinal calcium absorption
- Must maintain 25-OH vitamin D >50 nmol/L to exclude this cause 1
- Requires long-term correction (>6 months) before diagnosing PHPT 1
Gastrointestinal Disorders:
- Malabsorption syndromes
- Inflammatory bowel disease
- Bariatric surgery (induces vitamin-calcium malabsorption) 5, 3
Medications:
- Loop diuretics (furosemide): Increase urinary calcium excretion; in a 70-kg patient, 40 mg daily furosemide increases PTH by 22.8 pg/mL 6
- Inadequate calcium supplementation: 500 mg elemental calcium decreases PTH by 8.7 pg/mL 6
- Proton pump inhibitors: Impair calcium absorption 6
Hereditary Tubulopathies:
- Dent disease (X-linked recessive, CLCN5 gene mutation)
- Hereditary hypophosphatemic rickets with hypercalciuria (SLC34A3 gene)
- Fanconi-Bickel syndrome (Glut 2 transporter variants)
- These present with hypercalciuria, hypophosphatemia, nephrocalcinosis 5
Tertiary Hyperparathyroidism
Tertiary hyperparathyroidism develops after prolonged secondary hyperparathyroidism, where autonomous PTH secretion persists despite correction of the initial stimulus, resulting in hypercalcemia 4.
Typical Setting:
- Chronic kidney disease patients, especially post-kidney transplant
- Hypercalcemia with elevated PTH distinguishes this from secondary hyperparathyroidism 4
Spurious PTH Elevation (Laboratory Interference)
Critical pitfall: PTH can be falsely elevated due to assay interference, which must be considered when PTH levels are discordant with clinical presentation 7, 8.
Mechanisms:
- Heterophile antibodies: Most common cause of interference 7
- Macro-PTH complexes: PTH bound to immunoglobulins or other molecules 8
- Human anti-animal antibodies 7
Recognition:
- PTH levels unexpectedly high (often >300-700 pg/mL) with normal calcium, phosphate, vitamin D, and renal function 7, 8
- Diagnostic tests:
Differential Diagnosis Algorithm
Before diagnosing primary hyperparathyroidism, systematically exclude 5, 1:
- Vitamin D deficiency: Measure 25-OH vitamin D; supplement to >50 nmol/L for >6 months
- Renal insufficiency: Calculate eGFR
- Medications: Review thiazides, lithium, loop diuretics, bisphosphonates, denosumab
- Malabsorption: History of bariatric surgery, inflammatory bowel disease
Biochemical Patterns:
PTH elevated + Calcium elevated + Urinary calcium elevated:
- Primary hyperparathyroidism (most likely)
PTH elevated + Calcium elevated + Urinary calcium low (fractional excretion <2%):
- Familial hypocalciuric hypercalcemia (FHH) - genetic testing indicated 1
PTH elevated + Calcium normal:
- Secondary hyperparathyroidism (investigate causes above)
- Normocalcemic PHPT (diagnosis of exclusion) 1
PTH elevated + Calcium low:
- Severe vitamin D deficiency
- Chronic kidney disease
- Malabsorption
Special Diagnostic Tests
When diagnosis remains unclear after standard workup 5, 1:
- Oral calcium load test (Pak test): Assess calcium and PTH 2 hours after 1 gram oral calcium
- Cinacalcet test: 60 mg oral, measure calcium and PTH 1 hour later
- Pro-FHH score: Helps differentiate PHPT from FHH 1
Common Pitfalls
- Failing to correct vitamin D deficiency adequately before diagnosing PHPT - requires >6 months of normal levels 1
- Not recognizing assay interference when PTH is markedly elevated with otherwise normal biochemistry 7, 8
- Missing medication-induced causes, particularly loop diuretics 6
- Inadequate assessment of renal function - even mild CKD can cause secondary hyperparathyroidism 5
- Not screening for familial syndromes in young patients or those with multiglandular disease 3