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Now I have comprehensive content. Let me compile the full Harrison's answer on Hyperparathyroidism.
Hyperparathyroidism - Harrison's Principles of Internal Medicine (22nd Ed., 2025)
Overview & Classification
Hyperparathyroidism is broadly divided into:
- Primary - autonomous overproduction of PTH (adenoma, hyperplasia, carcinoma)
- Secondary - physiologic excess PTH due to hypocalcemic stimulus (e.g., chronic renal failure)
- Tertiary - autonomous PTH secretion arising from long-standing secondary hyperparathyroidism
Primary Hyperparathyroidism
Natural History & Incidence
Primary hyperparathyroidism is typically a disease of postmenopausal women. It results from excessive PTH secretion disproportionate to serum calcium, causing hypercalcemia and hypophosphatemia.
- Current US incidence: ~50 per 100,000 person-years
- Most patients today are diagnosed asymptomatically through routine multiphasic blood screening
- Classic presentation (nephrolithiasis, peptic ulcers, bone resorption, mental changes) is now less common
- Rare severe form: hypercalcemic parathyroid crisis - marked dehydration and coma
Etiology
| Cause | Frequency |
|---|---|
| Single adenoma | ~80% |
| Chief cell hyperplasia (all 4 glands) | ~15-20% (often hereditary) |
| Parathyroid carcinoma | rare |
| Multiple adenomas | rare |
| Ectopic PTH production | very rare |
Up to 10% of patients have a genetic basis for disease.
Hereditary Syndromes
| Syndrome | Features |
|---|---|
| MEN 1 (Werner's) | Hyperparathyroidism + pituitary tumors + pancreatic tumors; associated with Zollinger-Ellison syndrome |
| MEN 2A | Pheochromocytoma + medullary thyroid carcinoma + hyperparathyroidism |
| MEN 2B | Multiple neuromas; usually LACKS hyperparathyroidism |
| MEN 4 | Mutations in CDKN1B (p27); hyperparathyroidism + anterior pituitary tumors |
| HPT-JT Syndrome | Hyperparathyroidism-jaw tumor syndrome; mutations in CDC73/HRPT2 encoding parafibromin |
| FIHP | Familial isolated hyperparathyroidism; may represent variable expression of above syndromes |
Genetic Defects
Two fundamental types of genetic defects in parathyroid tumors:
- Overactivity of proto-oncogenes (gain-of-function, single allele sufficient)
- Loss of function of tumor-suppressor genes (both alleles required; biallelic loss)
Key genes:
- MEN1 gene (chromosome 11q13) - encodes tumor suppressor MENIN. Germline + somatic "second hit" causes monoclonal expansion (Knudson two-hit hypothesis). Also somatically deleted in 15-20% of sporadic adenomas
- RET oncogene - gain-of-function mutations in MEN 2
- CDC73/HRPT2 (chromosome 1q21-31) - encodes parafibromin (531 amino acids); inactivating mutations cause parathyroid carcinoma and HPT-JT syndrome. Notably, HRPT2 mutations are not seen frequently in benign adenomas - parathyroid carcinoma follows a different genetic pathway than adenomas
Clinical Features of Hypercalcemia (all causes)
Symptoms typically appear at calcium >2.9-3.0 mmol/L (>11.6 mg/dL):
| System | Manifestations |
|---|---|
| General | Fatigue, depression, mental confusion, anorexia |
| GI | Nausea, vomiting, constipation, peptic ulcers |
| Renal | Polyuria, nephrolithiasis, reversible tubular defects, renal insufficiency |
| Cardiac | Short QT interval, arrhythmias |
| Musculoskeletal | Bone resorption (osteitis fibrosa cystica in severe/chronic cases) |
At calcium >3.2 mmol/L (>12.8 mg/dL): calcification in kidneys, skin, vessels, lungs, heart, stomach.
At calcium ≥3.7-4.5 mmol/L (>14.8 mg/dL): severe emergency - coma, cardiac arrest.
Diagnosis
- PTH assay: Second-generation double-antibody immunometric (intact PTH) assays are the standard - eliminate interference from biologically inactive fragments. Third-generation assays (detecting even fewer fragments) are useful in chronic renal disease research but have not replaced second-generation
- Serum phosphate: Usually low, may be normal if renal failure is present
- Albumin-corrected calcium: Confirm true hypercalcemia; obtain multiple samples to avoid false positives from hemoconcentration or elevated serum proteins
Causes of Hypercalcemia (Differential Diagnosis)
I. Parathyroid-Related
- Primary hyperparathyroidism (adenoma, MEN, carcinoma, ectopic PTH)
- Lithium therapy
- Familial hypocalciuric hypercalcemia (FHH)
II. Malignancy-Related
- Osteolytic metastases (breast, myeloma, lymphoma)
- Humoral hypercalcemia of malignancy - PTHrP (squamous cell lung, kidney, breast)
- 1,25(OH)2D-mediated (lymphoma, ovarian dysgerminoma)
III. Vitamin D-Related
- Vitamin D intoxication (usually requires >10,000 U/d)
- Elevated 1,25(OH)2D: sarcoidosis, granulomatous diseases (macrophages convert 25-OHD to 1,25-OHD autonomously, unregulated by PTH)
IV. High Bone Turnover
- Hyperthyroidism, immobilization, thiazides, vitamin A intoxication, fat necrosis
V. Associated with Renal Failure
- Tertiary hyperparathyroidism, aluminum intoxication, milk-alkali syndrome
Treatment: Primary Hyperparathyroidism
Surgery (Definitive)
Parathyroidectomy is the definitive therapy. Surgery is increasingly favored even in mild/asymptomatic cases due to growing evidence of skeletal, cardiovascular, and neuropsychiatric complications.
Surgical approaches:
- Conventional parathyroidectomy: Neck exploration under general anesthesia
- Minimally invasive parathyroidectomy: Outpatient, local anesthesia - gaining traction
Preoperative imaging:
- Neck ultrasound
- 99mTc-sestamibi SPECT
- C(11)-choline PET/CT
- 4D-CT
Intraoperative PTH monitoring: Rapid PTH immunoassay - a >50% fall in PTH to normal levels indicates successful removal.
Special surgical considerations:
- Familial cases: multiple-gland hyperplasia increases risk of permanent hypoparathyroidism
- Transplantation of minced parathyroid tissue into forearm muscles (for easier re-excision if recurrence)
- Undiscovered adenomas: can be in mediastinum, or patient may have 5-6 glands
Post-op hypocalcemia:
- Serum calcium typically falls within 24h and reaches low-normal for 3-5 days
- "Hungry bone syndrome" - undermineralized bone avidly absorbs calcium
- Exacerbated by coexistent vitamin D deficiency or hypomagnesemia
Medical Surveillance (Asymptomatic Disease)
Still preferred by some physicians and elderly patients with mild disease. Close monitoring of calcium, renal function, and bone density is required.
Calcimimetics (cinacalcet): Reduces PTH and serum calcium in patients who are not surgical candidates, though does not improve bone density.
Bisphosphonates: Used to protect bone density in medically managed patients.
Secondary Hyperparathyroidism
Occurs due to chronic hypocalcemic stimulation, most commonly from chronic renal failure. PTH secretion is a physiologic response attempting to restore normocalcemia.
Tertiary Hyperparathyroidism
Develops when long-standing secondary hyperparathyroidism leads to autonomous PTH hypersecretion, even after correction of the underlying cause (e.g., post-renal transplant). The glands become independently hyperactive, causing hypercalcemia.
Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Chapter 422 - Diseases of the Parathyroid Gland and Other Hyper- and Hypocalcemic Disorders, pp. 3318-3322
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