This document summarizes the key components involved in vitamin D and calcium homeostasis, including the roles of vitamin D, parathyroid hormone (PTH), and calcitonin. It discusses how vitamin D is obtained from sunlight or diet and activated in the body. PTH and calcitonin work in opposition to regulate calcium levels - PTH increases calcium levels by acting on the kidneys and bone, while calcitonin decreases calcium levels by inhibiting bone resorption. PTH and vitamin D also interact, as vitamin D helps absorb calcium from the gut and PTH stimulates its production. Together this maintains appropriate calcium and phosphate levels in the blood and body.

1. 7-dehydrocholesterol Previtamin D3
Solar UVB radiation
Skin
Vitamin D3
Diet
25-hydroxylase
25(OH)-D
1-OHase
1,25(OH)2-D
PTH effects on the kidney
1.Stimulates 1-OHase.
2.Decreases PO4 resorption in proximal tubule via
increase in intracellular (and urinary) cAMP.
3.Increases Ca resorption in distal tubule.
1,25(OH)2-D effects on kidney
Increases Ca resorption in distal
tubule, by upregulating calbindin.
1α-hydroxylation
Stimulated by Inhibited by
PTH 1,25(OH)2-D
Low PO4 High PO4
Possibly GH and insulin
Extra-renal 1α-hydroxylation in
macrophages, e.g. in sarcoidosis.
Dietary Vit D
Average daily intake 500 iU.
Recommended intake 400 iU.
25(OH)-D
25-hydroxylase activity is essentially
unregulated. 25(OH)-D is the major
circulating metabolite, acting as a store.
PTH effects on bone
Acts on osteoblast cell
membrane to increase
permeability to calcium,
activating a pump which
drives calcium into ECF
PTH production
Main stimulus is low Ca2+.
Inhibited by 1,25(OH)2-D.
Mg required for synthesis,
release and action of PTH.
Response to hypocalcaemia
Rise in PTH, causing:
• increased renal Ca absorption;
• increased renal PO4 excretion;
• mobilization of calcium and PO4 from bone;
• increased 1α-hydroxylation, causing:
• increased GI absorption of calcium and PO4.
Response to hypophosphataemia
Stimulates 1α-hydroxylation, causing:
• increased GI absorption of PO4 and calcium;
• rise in calcium inhibits PTH release, causing:
• increased renal PO4 resorption;
• reduced renal calcium resorption.
Vitamin D replacement
Ergocalciferol = Vit D2
Cholecalciferol = Vit D3
Alfacalcidol = 1α(OH)-D3
Calcitriol = 1,25(OH)-D3
Calcitonin production
Main stimulus is high
Ca2+ (ionized>1.15) –
also glucagon, gastrin,
ß-adrenergic agonists (?
reason for low Ca in
acute illness)
Calcitonin effects on bone
• inhibits osteoclast resorption
• thereby lowers Ca and PO4
• no effect on Mg
1,25(OH)2-D effects on gut
Increases Ca2+
and PO4
absorption by upregulating
intracellular calbindin
proteins.
Vit D effects
on bone?
Unsure if direct
effect
Calcitonin
Parathyroid
hormone