Phase 1: Early CKD (G3a, eGFR 45–59)

Trigger: Nephron loss → ↓ filtered load of phosphate

Compensatory response: 1. FGF23 ↑↑ (fibroblast growth factor 23) - Secreted by osteoblasts/osteocytes in response to ↓ GFR + ↑ phosphate - Early marker of dysregulation; may ↑ before serum PO4 rises - Suppresses PTH, ↑ FGF23-mediated phosphate excretion - Inhibits 1,25-(OH)2 vitamin D (active vitamin D) synthesis

2. PTH rises slightly (phosphate retention drives PTH)
   • ↑ Serum PTH → ↑ phosphate excretion per remaining nephron
   • Net effect: Serum PO4 stays NORMAL for longer
3. Serum phosphate remains NORMAL (~3.5–4.5 mg/dL)

Phase 2: Moderate-to-Advanced CKD (G3b–G4, eGFR 15–44)

Worsening phosphate retention: - More nephrons lost → even with ↑↑ FGF23, phosphate accumulates - Serum phosphate begins to rise (>4.5 mg/dL)

PTH escalation: - Combines effects of: hyperphosphatemia + ↓ 1,25(OH)2 vitamin D (low calcium) + persistent FGF23 - PTH ↑↑↑ (may exceed 100 pg/mL)

Vitamin D metabolism disrupted: - ↓ 1,25-(OH)2 vitamin D (active form) - Normally produced by proximal tubule with FGF23 suppression - FGF23 ↑↑ → ↓↓ 1,25(OH)2D synthesis - ↓ 1,25(OH)2D → ↓ intestinal calcium absorption → ↓ serum Ca → ↑ PTH (vicious cycle)

• ↑ 25-OH vitamin D may be normal — stored form less affected (but may be low from poor nutrition/sun exposure)

Result: Secondary hyperparathyroidism (2° HPT) — PTH elevated due to CKD-induced stimuli

Phase 3: Advanced CKD & ESRD (G5, eGFR <15)

Overt hyperphosphatemia: PO4 >5 mg/dL

Tertiary hyperparathyroidism: PTH becomes autonomous - Even if phosphate, calcium normalize, PTH remains ↑ (from years of stimulation + parathyroid gland hyperplasia)

Calcification cascade: - Ca-PO4 product > 55 (risk zone for precipitation) - Calcification occurs in: coronary arteries, aorta, cardiac valves, soft tissues - Vascular calcification → arterial stiffness → ↑ pulse pressure → LVH → HF

Bone disease spectrum: 1. High-turnover bone disease (secondary HPT) - Excessive PTH → osteoblast & osteoclast activity ↑ - Rapid remodeling, loss of cortical bone, trabecular preservation 2. Low-turnover bone disease (adynamic bone) - Suppressed PTH (over-treatment with vitamin D, calcimimetics) - Minimal bone remodeling → impaired fracture healing 3. Mixed uremic osteodystrophy — elements of both

When to Start Monitoring

Key Lab Parameters

CKD G3a–G3b (eGFR 30–59)

Goal: Prevent secondary HPT development

1. Phosphate restriction
   • Diet: <1,000 mg/day (limit dairy, nuts, processed foods)
   • No binders needed yet if serum PO4 normal
2. Vitamin D repletion (if 25-OH VitD <30)
   • Ergocalciferol (Vitamin D2) 50,000 IU weekly × 12 weeks, then monthly
   • OR Cholecalciferol (Vitamin D3) 1,000–2,000 IU daily
   • Goal: 25-OH VitD 30–100 ng/mL
3. Monitor PTH trend
   • If PTH ↑ into 65–110 range → consider active vitamin D
4. Calcium neutral approach — avoid hypercalcemia

CKD G4 (eGFR 15–29)

Goal: Control PTH, phosphate; prevent vascular calcification

1. Phosphate binders (if PO4 >4.5 mg/dL)
   • Calcium-based: Calcium carbonate, calcium acetate
     • Pro: Inexpensive, help with acidosis
     • Con: ↑ Calcium load → vascular calcification risk if Ca-PO4 product >55
   • Non-calcium: Sevelamer, lanthanum, sucroferric oxyhydroxide
     • Pro: Don’t ↑ Ca; may ↓ phosphate absorption better
     • Con: Expensive, GI tolerability, multiple daily doses
   Key Point

   Modern trend: Avoid calcium-based binders in late CKD; prefer non-calcium agents to minimize Ca load and vascular calcification risk.

2. Active vitamin D (if PTH >110 AND Ca-PO4 product <55)
   • Calcitriol (1,25-(OH)2D3): Start 0.25–0.5 mcg daily–BID (potent; monitor Ca/PO4)
   • Paricalcitol (1,25-(OH)2D analogue): 1–2 mcg daily (selectivity for VDR in parathyroid)
   • Goal: Suppress PTH while maintaining Ca/PO4 balance
3. Calcimimetics (if PTH severely ↑ or hypercalcemia)
   • Cinacalcet: Allosteric activator of CaSR → ↓ PTH secretion
   • Doesn’t ↑ serum Ca or PO4 (advantage over vitamin D)
   • Use if PTH >300 despite binders + vitamin D, or if hypercalcemia
4. Monitor: Ca, PO4, PTH monthly

CKD G5 (eGFR <15, Dialysis)

Goal: PTH in target range (150–300 pg/mL); prevent hyperphosphatemia, calcification

1. Dialysis phosphate removal
   • Depends on GFR, urine output, residual kidney function
   • ~70–80% of phosphate removed by dialysis
2. Aggressive phosphate management
   • Binders with meals: Non-calcium preferred (sevelamer, lanthanum)
   • Dietary PO4 restriction <1,000 mg/day
   • Avoid high-phosphate foods (colas, processed meats, dairy)
3. PTH target range: 150–300 pg/mL (KDIGO 2009)
   • Some argue 200–400 acceptable (more physiologic; prevents adynamic bone)
   • Avoid over-suppression (adynamic bone, vascular stiffness)
4. Calcium balance
   • Avoid hypercalcemia (Ca >10 mg/dL)
   • Consider low-Ca dialysate if hypercalcemia
5. Bone biopsy (if unexplained bone pain, fractures, or concern for adynamic bone)
   • Tetracycline labeling → assess turnover, mineralization
   • Guides intensity of therapy

FGF23 Antagonists (Research phase)

• Pegpamorelin, others in development
• Target upstream: Block FGF23 directly
• May prevent secondary HPT earlier
• Not yet routine but promising

Phosphate Binders (Newer generation)

• Sucroferric oxyhydroxide — once-daily dosing (better adherence)
• Patiromer (potassium binders): Also binds phosphate; dual benefit if hyperkalemia

Screening for Osteoporosis in CKD

• DEXA (dual-energy x-ray absorptiometry) often NOT useful in CKD
  • May underestimate fracture risk (vascular calcification mimics high bone density)
  • Dynamic processes override static density measure
• Better approach: Clinical assessment
  • Age >70, female, steroid use, fracture history
  • Consider high-turnover vs low-turnover bone disease
  • PTH >300 → likely high turnover (less osteoporosis, more fracture from remodeling)

Management

1. Exercise & nutrition: Weight-bearing activity, adequate protein (1.0–1.2 g/kg)
2. Calcium balance: Avoid both hypo- and hypercalcemia
3. Vitamin D: Maintain 25-OH VitD >30 ng/mL
4. Avoid bisphosphonates in CKD G4–G5 — risk of low-turnover bone, impaired fracture healing
5. Consider anabolic agents (PTH analogues) if documented low-turnover bone on biopsy (rare, specialized)

Mechanisms of Vascular Calcification in CKD

1. Passive precipitation: Ca-PO4 product >55 → hydroxyapatite crystal formation in vessel walls
2. Active osteogenic conversion: Vascular smooth muscle cells transdifferentiate → osteoblast-like → produce mineralization matrix
3. Loss of calcification inhibitors: ↓ Fetuin-A, matrix Gla protein (MGP)
4. Oxidative stress & inflammation from uremia

Prevention

• Tight phosphate control (target <5.5 mg/dL at G5)
• Minimize Ca-PO4 product <55
• Avoid excess calcium supplementation
• Use non-calcium binders
• Manage inflammation (dialysis adequacy)