Bone disorders

Disorder	Osteoclast Activity	Osteoblast Activity	Mineralization	Key Associations
Osteoporosis	↑ (Resorption)	↓ (Formation)	Normal	Trabecular thinning, Normal Labs
Osteopetrosis	Dysfunctional (Defective resorption)	↔ / ↑	Normal	Thick bone, Carbonic Anhydrase II mut.
Osteomalacia / Rickets	↑ (Due to 2° PTH)	↑ (Reactive)	Defective	Unmineralized osteoid, soft bone, ↓ Vit D
Paget Disease	↑↑ (Lytic Phase)	↑↑ (Sclerotic Phase)	Normal	Mosaic pattern, woven bone, Isolated ↑ ALP
Osteitis Fibrosa Cystica t	↑↑ (PTH driven)	↑	Normal	Brown tumors, Subperiosteal thinning

Characterized by calcium level

• Elevated Calcium: Driven by systemic endocrine signaling (e.g., PTH, PTHrP, calcitriol) or extensive, cytokine-mediated osteoclast destruction surpassing renal clearance capacity.
• Normal Calcium: Driven by localized remodeling mismatches, isolated structural/collagen defects, or balanced architectural loss without excess net systemic mineral release.
• Disorders with Elevated Calcium (Hypercalcemia)
  • Primary Hyperparathyroidism (Osteitis Fibrosa Cystica): Autonomous parathyroid adenoma/hyperplasia → ↑ PTH-mediated osteoclast activation and renal reabsorption.
    • Labs: ↑ Ca, ↓ PO₄, ↑ PTH, ↑ ALP.
  • Humoral Hypercalcemia of Malignancy (HHM): Tumor secretion of PTHrP (squamous cell lung/head/neck, RCC).
    • Labs: ↑ Ca, ↓ PO₄, ↓ PTH, ↑ PTHrP.
  • Osteolytic Metastases & Multiple Myeloma (MM): Local cytokine-mediated osteoclast activation (IL-1, IL-6, RANKL) without osteoblastic repair.
    • Labs: ↑ Ca, ↓ PTH, normal ALP (osteoblast suppression in classic MM).
  • Granulomatous Disorders (Sarcoidosis, TB): Extra-renal macrophage 1α-hydroxylase production → autonomous calcitriol conversion.
    • Labs: ↑ Ca, ↑ PO₄, ↓ PTH, ↑ 1,25-(OH)₂D.
  • Prolonged Immobilization: Complete loss of mechanical stress in high-turnover states (young pts, Paget disease) → rapid osteoclast resorption.
    • Labs: ↑ Ca, ↓ PTH, ↑ urinary Ca (Rx: IV bisphosphonates).
  • Thyrotoxicosis: Excess T3/T4 directly stimulates osteoclastic resorption.
    • Labs: Mild ↑ Ca, ↓ PTH, ↑ ALP.
• Disorders with Normal Calcium (Eucalcemia)
  • Paget Disease of Bone (Osteitis Deformans): Localized, disorganized bone remodeling (osteolytic phase followed by chaotic osteoblastic deposition).
    • Labs: Normal Ca, normal PO₄, markedly ↑ ALP, normal PTH (Note: develops hypercalcemia only if immobilized).
  • Osteoporosis: Chronic, uncoupled architectural bone loss (resorption > formation) without acute systemic mineral release.
    • Labs: Normal Ca, normal PO₄, normal ALP, normal PTH (any lab abnormality warrants search for secondary cause).
  • Osteopetrosis: Defective osteoclast acidification pit (carbonic anhydrase II mutation) → impaired remodeling and dense, fragile bone.
    • Labs: Normal Ca, normal PO₄, normal PTH, normal/↑ ALP.
  • Osteogenesis Imperfecta: Defective type I collagen synthesis (COL1A1/COL1A2) affecting matrix structure rather than mineral metabolism.
    • Labs: Normal Ca, normal PO₄, normal ALP, normal PTH.
  • Fibrous Dysplasia: Replacement of normal bone/marrow with fibrous stroma and immature woven bone (GNAS mutation).
    • Labs: Normal Ca, normal PO₄, ↑ ALP, normal PTH.