Expansion
Parathyroid hormone and vitamin D raise calcium; calcitonin lowers it
Expansion
Parathyroid hormone (raises calcium, lowers phosphate)
- Bone: stimulates resorption, indirectly through osteoblast RANK ligand
- Kidney: increases calcium reabsorption, decreases phosphate reabsorption, and stimulates 1-alpha hydroxylase
- Gut: indirectly, through vitamin D
Calcitriol (1,25-dihydroxyvitamin D) (raises both calcium and phosphate)
- Gut: increases absorption of both, its main action
- Bone: permits mineralisation, and at high levels mobilises calcium
Calcitonin (lowers calcium)
- From thyroid parafollicular C cells; inhibits osteoclasts
- Physiologically minor in humans; neither thyroidectomy nor medullary carcinoma disturbs calcium significantly
The calcium-sensing receptor on parathyroid cells provides minute-to-minute control, suppressing hormone release when calcium rises.
Note that about half of plasma calcium is protein bound, so the ionised fraction is what matters. Correct for albumin, and remember that alkalosis increases binding and lowers ionised calcium, producing tetany without any change in total calcium.