7-dehydrocholesterol Previtamin D3
Solar UVB radiation
PTH effects on the kidney
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.
Stimulated by Inhibited by
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-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
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
Main stimulus is high
Ca2+ (ionized>1.15) –
also glucagon, gastrin,
ß-adrenergic agonists (?
reason for low Ca in
Calcitonin effects on bone
• inhibits osteoclast resorption
• thereby lowers Ca and PO4
• no effect on Mg
1,25(OH)2-D effects on gut
absorption by upregulating
Vit D effects
Unsure if direct