Calcium and Phosphate Metabolism

Ca & P Homeostasis - The Balancing Act

• Calcium (Ca): 99% in bone. Serum total: 8.5-10.5 mg/dL.
  • Ionized Ca (~50%) is physiologically active.
  • Bound to albumin (~40%), complexed (~10%).
• Phosphate (P): 85% in bone. Serum: 2.5-4.5 mg/dL.
  • Crucial for ATP, DNA/RNA, cell membranes.
• Dynamic balance: Maintained by PTH, Vitamin D, Calcitonin.
• Key sites: Bone (reservoir), kidney (excretion/reabsorption), gut (absorption).
• 📌 Ca x P product: Important for bone mineralization.

⭐ Ionized calcium (normal: 4.5-5.6 mg/dL or 1.12-1.4 mmol/L) is the critical fraction for neuromuscular excitability and cardiac function.

Parathyroid Hormone (PTH) - Calcium's Chief Conductor

• Source: Chief cells (parathyroid glands).
• Regulation:
  • Major: ↓ Serum $Ca^{2+}$ (via CaSR).
  • Minor: ↑ Serum $PO_4^{3-}$, ↓ $1,25(OH)_2D_3$.
  • $Mg^{2+}$: Severe ↓ inhibits PTH. 📌 "Magnesium is a drag, too low or too high, PTH will lag".
• Actions (Net: ↑ Serum $Ca^{2+}$, ↓ Serum $PO_4^{3-}$):
  • Bone: ↑ Resorption (↑ osteoclast activity).
  • Kidney:
    • ↑ $Ca^{2+}$ reabsorption (DCT).
    • ↓ $PO_4^{3-}$ reabsorption (PCT) -> phosphaturia.
    • ↑ 1α-hydroxylase -> ↑ $1,25(OH)_2D_3$.
  • Intestine (indirect): ↑ $Ca^{2+}$ & $PO_4^{3-}$ absorption (via Vit D).

actions on bone, kidney, and intestine to regulate calcium and phosphate)

⭐ PTH causes phosphaturia, a key mechanism to prevent calcium phosphate precipitation when mobilizing calcium from bone and increasing intestinal absorption.

Vitamin D - Sunshine Steroid

• Sources: Sunlight (D₃ - cholecalciferol), Diet (D₂ - ergocalciferol, D₃).
• Synthesis & Activation: 📌 Mnemonic (Activation): Skin (Sun) → Liver → Kidney.
  • Skin (UVB): 7-Dehydrocholesterol → Cholecalciferol (D₃).
  • Liver: D₃/D₂ → 25-Hydroxyvitamin D (Calcidiol) via 25-hydroxylase.
  • Kidney: Calcidiol → $1,25(OH)_2D_3$ (Calcitriol - active) via 1α-hydroxylase.
    • Stimulated by: PTH, ↓Ca²⁺, ↓PO₄³⁻.
    • Inhibited by: FGF-23, ↑Ca²⁺, ↑PO₄³⁻.
• Actions (Calcitriol):
  • Gut: ↑↑Ca²⁺ & ↑↑PO₄³⁻ absorption.
  • Kidney: ↑Ca²⁺ & ↑PO₄³⁻ reabsorption (synergistic with PTH for Ca²⁺).
  • Bone:
    • Mineralization (indirectly by ↑serum Ca²⁺/PO₄³⁻).
    • Resorption (direct on osteoblasts → stimulate osteoclasts; high doses).

⭐ Calcitriol is the most potent stimulator of intestinal calcium and phosphate absorption.

Calcitonin & FGF-23 - Fine-Tuning Factors

• Calcitonin:
  • Source: Thyroid C-cells (parafollicular).
  • Effect: ↓ serum $Ca^{2+}$; inhibits osteoclasts, ↑ renal $Ca^{2+}$ excretion.
  • Minor role in $Ca^{2+}$ homeostasis in humans.
  • 📌 Calcitonin tones down calcium.
• FGF-23:
  • Source: Osteocytes/osteoblasts.
  • Effect: ↓ serum $PO_4^{3-}$; ↓ renal $PO_4^{3-}$ reabsorption, ↓ 1α-hydroxylase activity (↓ active Vit D).
  • Needs Klotho co-receptor.

⭐ FGF-23 is a key phosphaturic hormone, also suppressing calcitriol production.

Mineral Imbalances - Clinical Snapshots

• Hypercalcemia (Ca > 10.5 mg/dL):
  • Symptoms: "Stones (renal), bones (pain), groans (abdominal), thrones (polyuria), psychiatric overtones".
  • ECG: Short QT interval.
• Hypocalcemia (Ca < 8.5 mg/dL):
  • Symptoms: Tetany (Chvostek's, Trousseau's signs), paresthesias, seizures. 📌 CATS (Convulsions, Arrhythmias, Tetany, Spasms).
  • ECG: Prolonged QT interval.
• Hyperphosphatemia (PO₄ > 4.5 mg/dL):
  • Often asymptomatic; chronic: vascular calcification, soft tissue deposits.
• Hypophosphatemia (PO₄ < 2.5 mg/dL):
  • Muscle weakness, rhabdomyolysis, respiratory failure, altered mental status.

⭐ Trousseau's sign (carpal spasm after sphygmomanometer cuff inflation) is more specific for hypocalcemia than Chvostek's sign (facial muscle twitch on tapping facial nerve).

High‑Yield Points - ⚡ Biggest Takeaways

• PTH ↑ serum Ca²⁺ & ↓ PO₄³⁻ via bone resorption & kidney effects.
• Vitamin D (Calcitriol) ↑ intestinal Ca²⁺ & PO₄³⁻ absorption.
• Calcitonin weakly ↓ serum Ca²⁺ by inhibiting osteoclasts.
• Primary Hyperparathyroidism: ↑ PTH, ↑ Ca²⁺, ↓ PO₄³⁻ ("stones, bones, groans").
• Hypoparathyroidism: ↓ PTH, ↓ Ca²⁺, ↑ PO₄³⁻, causing tetany.
• CKD causes Secondary Hyperparathyroidism: ↑ PTH, ↑ PO₄³⁻, variable Ca²⁺.
• FGF-23: Key phosphaturic hormone, inhibits Vitamin D activation.