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Human Body Study

Calcium–Phosphate Axis

The calcium–phosphate axis Parathyroid hormone from the parathyroid glands raises serum calcium via bone resorption, renal calcium reabsorption (with phosphate excretion), and activation of calcitriol (active vitamin D) in the kidney, which in turn increases gut absorption. Calcitonin from thyroid C-cells exerts a mild opposing effect. The calcium-sensing receptor on the parathyroids directly senses serum calcium and closes the feedback loop. Parathyroid glands chief cells · CaSR sensors Thyroid C-cells parafollicular cells PTH effects Bone: ↑ Ca²⁺ and P resorption Kidney: ↑ Ca²⁺ reabsorb, ↑ P excrete Kidney → ↑ calcitriol synthesis (active vitamin D) Gut: ↑ Ca²⁺ absorption (via calcitriol) Calcitonin effects Bone: ↓ osteoclast activity Kidney: ↑ Ca²⁺ excretion Mild Ca-lowering effect (minor in adult humans) PTH calcitonin ↑ serum Ca²⁺ ↓ serum Ca²⁺ Serum Ca²⁺ regulated variable ~8.5–10.5 mg/dL ↓ Ca²⁺ → PTH (CaSR) ↑ Ca²⁺ → calcitonin Calcitriol also suppresses PTH transcription (long-loop). FGF23 from bone regulates phosphate excretion. CaSR is the parathyroid calcium-sensing receptor — the direct sensor of serum Ca²⁺.

Signaling chain

Section titled “Signaling chain”

A pituitary-independent system coordinating three hormones across bone, kidney, gut, and the parathyroid glands:

  • PTH — released by the parathyroid glands when serum calcium falls.
  • Calcitriol (active vitamin D) — produced by renal 1α-hydroxylation of 25-OH vitamin D, upregulated by PTH.
  • Calcitonin — released by thyroid C-cells when calcium is high (minor role in humans).
  • FGF23 — bone-derived regulator of phosphate excretion.

Function

Section titled “Function”

Maintains tight serum calcium and phosphate homeostasis, essential for neuromuscular function, coagulation, and bone mineralization. PTH raises calcium by mobilizing bone, increasing renal calcium reabsorption (and phosphate excretion), and stimulating calcitriol. Calcitriol raises both calcium and phosphate by enhancing gut absorption.

Feedback

Section titled “Feedback”

Serum calcium is sensed directly by the parathyroid calcium-sensing receptor (CaSR); rising calcium suppresses PTH. Calcitriol also suppresses PTH transcription (long loop).

Clinical relevance

Section titled “Clinical relevance”
  • Primary hyperparathyroidism — high PTH, high calcium (adenoma); stones, bones, abdominal groans.
  • Hypoparathyroidism — low PTH, hypocalcemia, tetany.
  • Secondary hyperparathyroidism — compensatory, often from chronic kidney disease or vitamin D deficiency.
  • Vitamin D deficiency → rickets / osteomalacia.

Key labs

Section titled “Key labs”

Serum calcium (corrected/ionized), PTH, phosphate, 25-OH vitamin D, 1,25-OH vitamin D, magnesium, alkaline phosphatase.