Primary Hyperparathyroidism

Detailed Overview

It is the most common cause of hypercalcemia in the outpatient setting. PTH tightly regulates calcium homeostasis by acting on the bones, kidneys, and intestines. In PHPT, the normal negative feedback loop is lost. The classic phenotype of 'bones, stones, abdominal groans, and psychic moans' is rarely seen today; instead, >80% of patients are completely asymptomatic at diagnosis, discovered incidentally on routine blood panels. Despite being asymptomatic, chronic mild hypercalcemia can lead to silent nephrolithiasis and decreased cortical bone density (osteoporosis).

Epidemiology & Demographics

Prevalence is 1-4 per 1,000. Incidence peaks in the sixth decade of life. Females are affected 3 times more often than males, commonly occurring post-menopause.

Etiological Mechanism

Solitary parathyroid adenoma is responsible for 80-85% of cases. Multiple gland hyperplasia accounts for 10-15%. Parathyroid carcinoma is extremely rare (<1%). Most are sporadic, but ~5% are associated with MEN1 or MEN2A syndromes.

Primary Causes

Clonal proliferation of parathyroid chief cells forming an adenoma, often driven by somatic mutations in the PRAD1 (cyclin D1) or MEN1 genes.

• Age and Sex: Post-menopausal women have the highest risk.
• Neck Irradiation: History of radiation exposure to the head and neck in childhood or early adulthood.
• Lithium Therapy: Chronic lithium use can alter the set-point of the calcium-sensing receptor, causing hyperplasia.
• Multiple Endocrine Neoplasia: Germline mutations in MEN1 or RET (MEN2A) genes.

The parathyroid glands possess calcium-sensing receptors (CaSR). Normally, high serum calcium suppresses PTH release. In PHPT, an adenoma or hyperplastic glands have a raised 'set-point' or decreased sensitivity to calcium, continuously secreting PTH. Excess PTH acts directly on the osteoblasts, upregulating RANKL, which strongly activates osteoclasts, leading to bone resorption and calcium release. In the kidney, PTH increases calcium reabsorption in the distal tubule and decreases phosphate reabsorption, while also stimulating 1-alpha-hydroxylase to convert Vitamin D to its active form (calcitriol). Increased calcitriol enhances intestinal calcium absorption. The net result is hypercalcemia, hypophosphatemia, and hypercalciuria (because the filtered load of calcium eventually exceeds the tubular reabsorptive capacity, risking stone formation).

Characteristic Clinical Presentation

• Asymptomatic: Most common presentation; discovered on routine chemistry panel.
• Neuropsychiatric Symptoms: Mild depression, fatigue, lethargy, and difficulty concentrating ('brain fog').
• Nephrolithiasis: Flank pain and hematuria due to calcium oxalate or calcium phosphate kidney stones.
• Bone Pain: In severe cases, generalized bone pain from osteitis fibrosa cystica.

Physical Examination Signs

• Physical exam is typically completely normal (parathyroid adenomas are rarely palpable).
• Hypertension (mild correlation with hypercalcemia).
• Band keratopathy (calcium deposition in the cornea) - rare, seen only in severe chronic hypercalcemia.

• Osteoporosis: Particularly in cortical bone (e.g., distal radius), increasing fracture risk.
• Nephrolithiasis and Nephrocalcinosis: Calcium stones leading to obstruction, or parenchymal calcification causing chronic kidney disease.
• Peptic Ulcer Disease & Pancreatitis: Hypercalcemia stimulates gastrin secretion and pancreatic enzyme activation (rare modern presentations).

Diagnostic Criteria & Guidelines

Simultaneous finding of elevated total or ionized serum calcium WITH an elevated or inappropriately high-normal intact PTH level. Requires 24-hour urine calcium collection to rule out Familial Hypocalciuric Hypercalcemia (FHH).

Differential Diagnosis

• Malignancy-associated Hypercalcemia (PTHrP mediated)
• Familial Hypocalciuric Hypercalcemia (FHH)
• Vitamin D Intoxication
• Sarcoidosis / Granulomatous diseases

Laboratory Tests & Biomarkers

• Serum Calcium: Elevated (e.g., >10.5 mg/dL). Always correct for albumin or check ionized calcium.
• Intact Parathyroid Hormone (PTH): Elevated (e.g., >65 pg/mL) or inappropriately normal in the face of hypercalcemia.
• 24-hour Urine Calcium: Typically >200-250 mg/day in PHPT. If <100 mg/day, highly suspicious for FHH.
• Serum Phosphorus: Low or low-normal (<2.5 mg/dL).

Imaging Modalities & Findings

• Sestamibi Parathyroid Scan (Tc-99m): Radiotracer taken up by both thyroid and parathyroid, but washes out of the thyroid faster, leaving a 'hot spot' denoting the adenoma. Used purely for surgical localization, NOT for diagnosis.
• Neck Ultrasound: Hypoechoic nodule posterior to the thyroid gland.
• DEXA Scan: Reduced bone mineral density, typically worst at the distal 1/3 of the radius (cortical bone).

• Asymptomatic
  Mildly elevated calcium (<1.0 mg/dL above normal limit), no end-organ damage.
• Symptomatic / Target Organ Damage
  Presence of kidney stones, osteoporosis (T-score < -2.5), or impaired renal function.
• Hypercalcemic Crisis
  Calcium >14 mg/dL causing altered mental status, arrhythmias, and acute renal failure.

Second-Line & Adjunctive Therapy

For patients who are poor surgical candidates or refuse surgery: Cinacalcet (calcimimetic) 30 mg PO daily (lowers serum calcium but does not improve bone density). Bisphosphonates (e.g., Alendronate 70mg weekly) to improve bone density without significantly lowering serum calcium.

Surgical & Procedural Management

Minimally invasive targeted parathyroidectomy guided by pre-operative localization (Sestamibi/US) and confirmed with Intraoperative PTH monitoring (IOPTH). PTH has a half-life of 4 minutes; a >50% drop in IOPTH 10 minutes post-excision confirms cure.

Recommended Lifestyle Changes

• Maintain liberal fluid intake to dilute urine and prevent kidney stones.
• Maintain moderate calcium intake (1000 mg/day) - severe restriction is harmful as it stimulates more PTH release.
• Avoid thiazide diuretics, which decrease renal calcium excretion and worsen hypercalcemia.

Patient Counseling & Advice

Reassure patients that parathyroid tumors are almost never cancerous (>99% benign). Emphasize the importance of hydration. Clarify that restricting dairy products will not fix the problem and may worsen bone loss.

Follow-Up & Monitoring Schedule

For patients managed non-surgically: Serum calcium and creatinine every 6 months, and DEXA scan every 1-2 years. Post-operatively: monitor for transient hypocalcemia (perioral numbness, tingling) in the first 24-48 hours.

Preventive Strategies

No primary prevention available. Secondary prevention focuses on preventing kidney stones (hydration) and bone loss (vitamin D supplementation if deficient, as low vitamin D exacerbates PTH secretion).

Surgical cure rate is >95% when performed by an experienced endocrine surgeon. Post-operatively, bone mineral density improves significantly over 1-2 years, and kidney stone risk drops rapidly.