Calcium and Phosphate Homeostasis

Ca/P Overview - Mineral Dance

• Calcium (Ca²⁺): Total serum ~8.5-10.5 mg/dL (2.1-2.6 mmol/L).
  • Forms: Ionized (active, ~50%), protein-bound (~40%), complexed (~10%).
  • Functions: Bone, muscle contraction, nerve transmission, clotting.
• Phosphate (PO₄³⁻): Serum ~2.5-4.5 mg/dL (0.8-1.45 mmol/L).
  • Functions: Bone, ATP, DNA/RNA, cell membranes, acid-base buffer.
• Key Regulators: PTH, Vitamin D, Calcitonin. FGF-23 (phosphate).
• 📌 Mnemonic: "Calcium Contracts, Phosphate Powers".
• Corrected Ca (low albumin): $Ca_{corrected} = Ca_{measured} + \mathbf{0.8} \times (\mathbf{4} - Albumin)$.

⭐ Ionized Ca²⁺ is biologically active; critical for neuromuscular excitability.

PTH Deep Dive - Bone's Boss

• Source: Parathyroid chief cells.
• Regulation:
  • Primary trigger: ↓ Serum Ca²⁺ → ↑ PTH.
  • Also: ↑ Serum PO₄³⁻ (indirectly), ↓ Active Vit D → ↑ PTH.
  • Mg²⁺: Severe ↓ inhibits; mild ↓ stimulates PTH.
• Actions (Goal: ↑ Serum Ca²⁺, ↓ Serum PO₄³⁻):
  • Bone: ↑ Osteoclast activity (via osteoblast RANKL) → ↑ resorption.
    • Chronic ↑PTH: catabolic; Intermittent/low dose: anabolic.
  • Kidney:
    • ↑ Ca²⁺ reabsorption (DCT).
    • ↓ PO₄³⁻ reabsorption (PCT) → Phosphaturia (📌 PTH = Phosphate Trashing Hormone).
    • ↑ 1α-hydroxylase → ↑ Active Vitamin D.
  • Intestine (via ↑ Active Vit D): ↑ Ca²⁺ & PO₄³⁻ absorption.

mechanism of action on bone and kidney)

⭐ PTH significantly decreases phosphate reabsorption in the proximal convoluted tubule (PCT), leading to phosphaturia.

Vitamin D Path - Sunny Side Up

• Sources:
  • Skin (UVB): 7-DHC → Vit D3 (Cholecalciferol)
  • Diet: Vit D2 (Ergocalciferol), Vit D3 (Cholecalciferol)
• Activation Pathway:

• 1α-hydroxylase Regulation:
  • Stimulators: ↑ PTH, ↓ Serum $Ca^{2+}$, ↓ Serum $PO_4^{3-}$
  • Inhibitors: ↑ Calcitriol (feedback), ↑ FGF-23, ↑ Serum $Ca^{2+}$, ↑ Serum $PO_4^{3-}$
• Actions of Calcitriol ($1,25-(OH)_2D$):
  • Intestine: ↑ $Ca^{2+}$, $PO_4^{3-}$ absorption (primary site)
  • Bone: ↑ Mineralization (indirectly), ↑ Resorption (with PTH)
  • Kidney: ↑ $Ca^{2+}$, $PO_4^{3-}$ reabsorption
  • Parathyroid Gland: ↓ PTH synthesis, secretion

⭐ Calcitriol (1,25-(OH)2D) is the most potent stimulator of intestinal calcium absorption.

Calcitonin & Integration - Cool Down Crew

• Calcitonin: (📌 Calci-TONE-in TONES down $Ca^{2+}$)
  • Source: Thyroid parafollicular (C) cells.
  • Trigger: ↑ Serum $Ca^{2+}$.
  • Actions: Rapidly ↓ serum $Ca^{2+}$ & $PO_4^{3-}$.
    • Bone: Inhibits osteoclast activity (↓ resorption).
    • Kidney: ↑ $Ca^{2+}$ & $PO_4^{3-}$ excretion.
• Integrated Homeostasis:
  • PTH & Vitamin D: Dominant; ↑ $Ca^{2+}$ when levels are low.
  • Calcitonin: Opposes PTH; ↓ $Ca^{2+}$ when levels are high. Protects skeleton during $Ca^{2+}$ stress.

⭐ Calcitonin is a useful tumor marker for Medullary Thyroid Carcinoma (MTC).

High‑Yield Points - ⚡ Biggest Takeaways

• PTH ↑ serum Ca²⁺ & ↓ serum PO₄³⁻; stimulates bone resorption, renal Ca²⁺ reabsorption, & 1,25-(OH)₂D₃ synthesis.
• Calcitriol (Active Vitamin D) ↑ intestinal Ca²⁺ & PO₄³⁻ absorption; synergizes with PTH for bone resorption.
• Calcitonin ↓ serum Ca²⁺ by inhibiting osteoclast activity; its physiological role in humans is minor.
• Primary hyperparathyroidism features ↑ PTH, ↑ Ca²⁺, ↓ PO₄³⁻; remember "stones, bones, groans, psychiatric overtones".
• Hypoparathyroidism shows ↓ PTH, ↓ Ca²⁺, ↑ PO₄³⁻; presents with neuromuscular excitability (tetany).
• Vitamin D deficiency leads to ↓ Ca²⁺, ↓ PO₄³⁻, causing secondary hyperparathyroidism (↑ PTH).