Parathyroid Pathology — Hyperparathyroidism

• Primary Hyperparathyroidism — Adenomas: Most commonly (80-85% of cases), a single parathyroid adenoma demonstrates clonal expansion of chief cells with loss of CaSR sensitivity. Somatic mutations in the CCND1 gene (cyclin D1) and MEN1 gene (encoding menin) promote uncontrolled proliferation. The adenomatous cells secrete PTH autonomously regardless of serum calcium concentration, effectively resetting the set point for calcium suppression of PTH to pathologically higher levels.

• Hyperplasia (Primary): In 10-15% of primary cases, multiglandular hyperplasia affects all four parathyroid glands. Germline mutations in MEN1, MEN2A, CDC73 (parafibromin), and CASR (loss-of-function mutations causing familial hypocalciuric hypercalcemia [FHH]) predispose to this pattern. Hyperplastic glands demonstrate enlarged chief cell population with maintained architecture but increased cellularity.

• Carcinoma (Primary): Rare (<1% of primary cases), parathyroid carcinoma shows infiltrative growth, mitotic activity, capsular invasion, and vascular invasion. Often associated with CDC73 mutations. Produces massive PTH levels (often >3-4× normal) with severe hypercalcemia.

• Mechanism of Hypercalcemia: Elevated PTH directly stimulates osteoclastic bone resorption via RANK-L signaling on osteoblasts, increases renal tubular calcium reabsorption (distal convoluted tubule), and enhances 1,25-dihydroxyvitamin D production in the kidney, amplifying intestinal calcium absorption. The net effect is calcium mobilization exceeding renal excretory capacity.

• Secondary Hyperparathyroidism: Chronic hypocalcemia from renal failure or vitamin D deficiency drives persistent compensatory PTH elevation. Hyperplastic parathyroid tissue responds appropriately to stimuli but develops tertiary autonomous function after prolonged stimulation, with clonal expansion and altered CaSR expression.

• Primary Adenoma (85%): Sporadic occurrence; rare familial clustering via MEN1, MEN2A, or CDC73 mutations. Increased incidence with prior neck radiation exposure.

• Primary Hyperplasia (10-15%): MEN1 syndrome (parathyroid hyperplasia + pituitary adenoma + pancreatic islet tumors); MEN2A (parathyroid hyperplasia + medullary thyroid carcinoma + pheochromocytoma); Familial Hypocalciuric Hypercalcemia (FHH) from CASR inactivation causing abnormally high set point for PTH suppression; CDC73-associated hyperparathyroidism.

• Parathyroid Carcinoma (<1%): CDC73 mutations, prior neck radiation, PRUNE2 alterations. Often presents with extremely elevated PTH and severe symptomatic hypercalcemia.

• Secondary Hyperparathyroidism: Chronic kidney disease (most common; phosphate retention + decreased 1,25-vitamin D production), vitamin D deficiency, malabsorption, dietary calcium restriction.

• Tertiary Hyperparathyroidism: Evolution from long-standing secondary disease, typically in patients on hemodialysis >5 years or with severe renal osteodystrophy.

• Asymptomatic Hypercalcemia: 80% of patients with primary hyperparathyroidism are discovered incidentally on routine laboratory screening; no symptoms present. Represents early disease or mild elevation (serum calcium typically 10.5-11.5 mg/dL).

• Nephrolithiasis: Present in 15-20% of symptomatic patients. Mechanism involves PTH-induced hypercalciuria and hyperphosphaturia, creating supersaturated urine. Calcium phosphate and oxalate stones form; imaging reveals radiopaque calculi on KUB.

• Bone Disease ("Stones, Bones, Groans"): Chronic PTH excess stimulates osteoclastic resorption exceeding osteoblastic formation, resulting in osteoporosis or osteitis fibrosa cystica (in severe hyperparathyroidism). Pathological fractures, bone pain, and elevated alkaline phosphatase reflect increased bone turnover. Skeletal imaging shows subperiosteal resorption (especially radial aspect of fingers and distal clavicle—pathognomonic), salt-and-pepper osteoporosis, and brown tumors (benign lytic lesions of hyperparathyroid osteitis fibrosa cystica).

• Nephrogenic Diabetes Insipidus: PTH-mediated suppression of aquaporin-2 expression in collecting duct; results in polyuria, polydipsia, and nephrogenic nephrogenic DI. Correlates with chronicity of disease.

• Neuropsychiatric Manifestations: Cognitive slowing, personality changes, depression, anxiety. Severe acute hypercalcemia (>13 mg/dL) causes altered mental status, seizures, cardiac arrhythmias. Mechanism: hypercalcemia decreases neuronal excitability threshold and prolongs QT interval.

• Gastrointestinal Symptoms ("Groans"): Nausea, vomiting, peptic ulcer disease (PTH stimulates gastrin secretion), pancreatitis (direct effect of hypercalcemia on pancreas).

• Hypertension: Present in 50% of hyperparathyroid patients; mechanism involves increased intracellular calcium in vascular smooth muscle and PTH-mediated sympathetic activation.

• Hypophosphatemia: Serum phosphate typically 2.0-3.0 mg/dL (normal 2.5-4.5). PTH inhibits sodium-phosphate cotransporter (NaPi-IIa) in proximal tubule, causing phosphate wasting.

• Biochemical Diagnosis: Elevated serum PTH with hypercalcemia is the hallmark of primary hyperparathyroidism. Ionized calcium (preferred) or total calcium corrected for albumin >10.5 mg/dL; PTH typically 65-200 pg/mL (normal 10-65). 24-hour urine calcium helps classify: elevated in primary adenoma/hyperplasia (filtered load exceeds reabsorption); paradoxically low-normal in FHH (CaSR defect causes increased renal calcium reabsorption). Serum phosphate is suppressed (2.0-3.0 mg/dL). 1,25-dihydroxyvitamin D is elevated or inappropriately normal despite hypercalcemia.

• Histological Findings (Post-operative Specimen):
• Adenoma: Hypercellular mass composed predominantly of chief cells (small, dark nuclei, sparse cytoplasm) or oxyphil cells (abundant mitochondria, granular pink cytoplasm). Rim of normal atrophic parathyroid tissue at periphery is diagnostic feature. Intact fibrous capsule. Lacks mitotic activity, atypia, or necrosis (distinguishes from carcinoma).
• Hyperplasia: Enlargement of all four glands with increased chief cell cellularity and decreased fat content. Preserved glandular architecture (acini separated by thin fibrous septa). Absence of capsule around individual nodules.
• Carcinoma: Infiltrative borders, increased mitotic figures (>5 per 10 high-power fields), tumor necrosis, capsular invasion, vascular invasion. May show nuclear enlargement and irregularity (atypia).

• Gross Pathology: Adenomas are typically 0.5-5 cm, tan-brown to reddish, with soft to firm consistency. Hyperplastic glands are mildly to markedly enlarged (can exceed 5 grams total combined weight). Carcinomas are hard, infiltrative, often gray-white, frequently with local invasion into surrounding soft tissue or thyroid.

• Imaging Studies:
• Sestamibi scan (Tc-99m): Most specific for localization; adenomas/hyperplasia retain tracer longer than normal thyroid, showing "dual-phase" pattern (early uptake in thyroid and parathyroid glands; delayed clearance from parathyroid). Sensitivity 90% for adenomas, lower for multiglandular disease.
• Ultrasound: Hypoechoic or isoechoic nodule; sensitivity 70-85% for adenomas. Less useful for hyperplasia.
• 4D CT: High sensitivity for multiglandular disease and ectopic adenomas; shows adenomas as hyperdense lesions on delayed phase.
• Intraoperative PTH monitoring: Confirms adequate resection; PTH should drop >50% from baseline within 10 minutes of adenoma removal (guides surgeon to extent of resection).

• Diagnostic Criteria:
• Primary hyperparathyroidism: Elevated serum PTH + hypercalcemia ± elevated 1,25-vitamin D
• Secondary hyperparathyroidism: Elevated PTH + hypocalcemia (or normal calcium) + low 1,25-vitamin D (or high in early renal disease)
• FHH: Elevated PTH + hypercalcemia + low 24-hour urine calcium (<200 mg/day; usually <100) + positive family history; CASR gene testing confirms diagnosis

• Asymptomatic Primary Hyperparathyroidism (Surgery vs. Observation):
• Surgical parathyroidectomy is curative and recommended if: serum calcium >1.0 mg/dL above upper normal limit, 24-hour urine calcium >400 mg/day, creatinine clearance reduced (GFR <60), T-score ≤-2.5 on bone density study (osteoporosis), patient age <50, or inadequate follow-up likely. Gold standard is bilateral neck exploration (allows identification/biopsy of all four glands) or directed unilateral exploration with intraoperative PTH monitoring (if preoperative imaging clearly identifies single adenoma).
• Medical observation acceptable for asymptomatic patients with mild hypercalcemia, normal renal function, and normal bone density; requires regular monitoring of serum calcium, creatinine, and bone density annually.

• Symptomatic Primary Hyperparathyroidism:
• Parathyroidectomy is definitive; removes source of PTH. Cure rate >95% for adenomas, 90-95% for hyperplasia (requires subtotal resection or total resection with autotransplantation, typically to sternocleidomastoid muscle). Intraoperative PTH assay confirms adequate resection.
• Preoperative localization via sestamibi scan, ultrasound, or 4D CT guides extent of exploration.

• Acute Severe Hypercalcemia (>13 mg/dL with symptoms):
• Aggressive intravenous saline hydration (0.9% NS, 200-500 mL/hr) to increase glomerular filtration and promote calciuria; monitor urine output and electrolytes closely.
• Loop diuretics (furosemide 40-80 mg IV) after volume repletion to further enhance urinary calcium excretion.
• Bisphosphonates (zoledronic acid 4-8 mg IV or pamidronate 60-90 mg IV) inhibit osteoclastic resorption; onset 2-4 days; effect lasts weeks.
• Calcitonin (4-8 units/kg SC/IV q6h) rapidly inhibits osteoclast function and increases renal calcium wasting; onset within hours but short duration (1-2 days); useful bridge therapy.
• Corticosteroids (prednisone 40-60 mg daily) in cases of malignancy-related hypercalcemia or granulomatous disease; inhibit 1,25-vitamin D production.
• Cinacalcet (calcimimetic agent) enhances CaSR sensitivity in parathyroid glands; reduces PTH and calcium; useful for tertiary hyperparathyroidism or patients awaiting surgery.
• Definitive treatment: Parathyroidectomy (for primary disease) or management of underlying cause (malignancy, granuloma, renal disease).

• Secondary Hyperparathyroidism:
• Vitamin D repletion (if deficient); cholecalciferol 50,000 units weekly or calcitriol 0.25-0.5 mcg twice daily.
• Phosphate binders (calcium carbonate, sevelamer, lanthanum) to lower serum phosphate and reduce PTH stimulation; especially critical in renal failure.
• Parathyroidectomy or cinacalcet for refractory tertiary hyperparathyroidism (PTH >800-1000 pg/mL despite medical management).

• Monitoring Post-operatively: Serial serum calcium at 4-6 weeks; assess for postoperative hypocalcemia ("hungry bone syndrome"—rapid PTH drop allows osteoblasts to remineralize skeleton, consuming serum calcium). May require calcium and vitamin D supplementation temporarily.

• Nephrolithiasis: Occurs in 15-20% due to hypercalciuria and hyperphosphaturia. Recurrent stones can cause chronic kidney disease and renal insufficiency. Mechanism: PTH increases urinary calcium and phosphate, creating supersaturated urine favoring precipitation.

• Osteitis Fibrosa Cystica: Severe hyperparathyroidism causes intense osteoclastic resorption with replacement by fibrotic connective tissue and hemorrhage, creating characteristic brown tumors (benign cystic lesions). Results in skeletal deformity, pathological fractures, and severe morbidity. Largely prevented by earlier diagnosis and treatment today.

• Neuropsychiatric Sequelae: Chronic cognitive impairment, depression, personality changes. Severe acute hypercalcemia causes seizures, coma, cardiac dysrhythmias (prolonged QT → torsades de pointes), potentially fatal.

• Nephrogenic Diabetes Insipidus: PTH suppresses aquaporin-2 expression; polyuria, polydipsia, volume depletion. Usually reversible with parathyroidectomy.

• Acute Pancreatitis: Direct effect of hypercalcemia on pancreatic acinar cells; may

The stem-defining facts

• PTH level is the single best next step in any hypercalcemia stem: after confirming with albumin-corrected or ionized calcium, an inappropriately normal or high PTH means PTH-dependent disease (adenoma, hyperplasia, FHH, lithium, thiazide); a suppressed PTH points to malignancy (PTHrP, osteolytic metastases, 1,25-D from lymphoma), granulomatous disease, or vitamin D toxicity.
• Imaging never makes the diagnosis: sestamibi, ultrasound, and 4D-CT localize the culprit gland for the surgeon. The AAES guideline on definitive management of primary hyperparathyroidism frames localization as a pre-operative planning tool only — ordering a scan before biochemical confirmation is the classic distractor.
• FHH is the trap: hypercalcemia + high-normal PTH + low urinary calcium (calcium/creatinine clearance ratio characteristically below ~0.01) + family history since childhood. Loss-of-function CASR mutation. Parathyroidectomy does not cure it — do not operate.

Buzzwords that map to one answer

• Subperiosteal resorption of the radial aspect of the middle phalanges, salt-and-pepper skull, distal clavicle tapering, and brown tumors (osteoclasts, fibrous stroma, hemosiderin) = osteitis fibrosa cystica.
• Single hypercellular gland with a rim of compressed atrophic parathyroid = adenoma; all four glands enlarged = hyperplasia, and hyperplasia should trigger a search for MEN1 (pituitary/pancreas) or MEN2A — in MEN2A, exclude pheochromocytoma before any neck operation.

Distinguishing the three types by labs

• Primary: ↑PTH, ↑Ca, ↓phosphate. Secondary (CKD): ↑PTH, ↓/normal Ca, ↑phosphate — the phosphate direction is the discriminator. Tertiary: ↑↑PTH with hypercalcemia in a long-standing dialysis patient; KDIGO CKD-MBD guidance favors calcimimetic, active vitamin D, or phosphate binders before parathyroidectomy.

Common distractors

• Hypercalcemia shortens the QT interval; prolongation belongs to hypocalcemia.
• Post-parathyroidectomy tetany with low calcium, low phosphate, and high alkaline phosphatase = hungry bone syndrome, not surgical hypoparathyroidism.
• Cortical bone (distal one-third radius) is lost preferentially, so DXA must include that site.